In short, for 120 League Medals you can get

- ... 72 hours (Builder Potions) or

- ... up to 64 hours (Hammer of Building) or

- ... up to ~24 hours (Hammer of Heroes)

closer to finishing all your Buildings, Traps and Heroes or

- ... 276 hours (Research Potions) or

- ... up to 384 hours (Hammer of Fighting) or

- ... up to 384 hours (Hammer of Spells)

closer to finishing all your Troops, Spells and Siege Machines.

Disclaimer:

I'll assume you use 6 Builders and only 1 Researcher. In case you use less than 6 Builders, the Hammers of Heroes and of Building will become more useful. In case you use 7 Builders (Goblin Builder as 7th Builder), Builder Potions will get slightly (1/6) more useful, but the Goblin will also take a lot more of your Gems over the course of the event when using Builder Potions! In case you use 2 Researchers (Goblin Researcher), Research Potions will become twice as strong. But the Goblin Researcher will also take a whole lot more of your Gems over the course of the event when using these Potions!

Detailed calculations:

1. What takes longer (Builder or Laboratory Time)?

1.1. First of all, all Buildings, Traps and Heroes from Town Hall 1 up to Town Hall 16 take less than 1 year 5 months when using 6 Builders In reality, it takes a while to unlock all 6 Builders. Assume it takes 6 months, then we would be at 1 year 6 months Builder Time until maxed Town Hall 16.

Source (Clash Ninja): https://www.clash.ninja/guides/how-long-to-upgrade-th-2-to-16

1.2. However, all Laboratory upgrades accumulated take about 2 years and 9 months until maxed Town Hall 16.

Source (Clash Ninja): https://www.clash.ninja/guides/how-long-to-upgrade-th-2-to-16

1.3. Therefore, I would argue that Research buffs are more important for most players than Builder buffs.

2. Comparing Potions with Hammers

2.1. Builder Potions vs Hammers of Building/Heroes

2.1.1. Builder Potions (30 League Medals) grant 9h reduction per Builder. The Hammerjam event doubles that, increasing it to 18h, effectively. Assuming 6 Builders, one Builder Potion gives 108h (4.5 days) of time reduction.

2.1.2. Hammer of Building (120 League Medals) varies in usefulness, depending on how long your longest available Building upgrade takes. It can save you up to 384h (16 days) at Town Hall 16. This does not change through the Hammer Jam event.

2.1.3. Hammer of Heroes (165 League Medals) also varies in usefulness as Heroes' upgrade times increase with higher Town Hall Levels. It can save you up to 192 hours (8 days) from Town Hall 15 on. This doesn't change through the Hammer Jam event either.

2.1.4. To compare the value of those 3, we "normalise" them to what we'd get for 120 League Medals:

- 4 × Builder Potions: 432h (18 days)

- 1 × Hammer of Building: 384h (16 days)

- 24/33 (~0.73) × Hammer of Heroes: ~140h (5.83 days)

2.2. Research Potions vs Hammers of Fighting/Spells

2.2.1. Research Potions (20 League Medals) grant 23h reduction per Researcher. The Hammerjam event doubles that, increasing it to 46h, effectively. Assuming 1 Researcher (no Goblin Researcher), one Research Potion gives 46h (~1.917 days) of time reduction.

2.2.2. Hammer of Fighting (120 League Medals) varies in usefulness, depending on how long your longest available Troop/Siege Machine upgrade takes. It can save you up to 384h (16 days) at Town Hall 16. This does not change through the Hammer Jam event.

2.2.3. Hammer of Spells (120 League Medals) also varies in usefulness, depending on how long your longest available Spell upgrade takes. It can save you up to 384h (16 days) at Town Hall 16. This doesn't change through the Hammer Jam event either.

2.2.4. To compare the value of those 3, we "normalise" them to what we'd get for 120 League Medals:

- 6 × Research Potions: 276h (11.5 days)

- 1 × Hammer of Fighting: 384h (16 days)

- 1 × Hammer of Spells: 384h (16 days)

3. Comparing Builder Time with Research Time

3.1. Simply adding up total time is not suitable to compare Builder to Research Time because with 6 Builders we can work 6 times as fast on Buildings as we can on our Lab. Therefore, I will divide the Builder Time by 6. Now, the results will show the real time my village would've taken to get a certain amount of upgrades done outside any events and without the Goblin Workers.

3.2. The results are the ones I put at the very top of this post. Ranked according to efficiency, extrapolated to being worth 120 Clan League Medals:

- Hammer of Fighting: 384h (16 days)

- Hammer of Spells: 384h (16 days)

- Research Potions: 276h (11.5 days)

- Builder Potions: 72h (3 days)

- Hammer of Building: 64h (~2.67 days)

- Hammer of Heroes: ~24h (~1 day)

3.3. At this point, I want to remind you that in order to max out your village your Laboratory will take longer than your Builders.

3.3.1. Therefore, purchasing Builder Potions or Hammers of Building/Heroes isn't recommended, even in the current Hammer Jam event! Of course, every player has a different playstyle though.

3.3.2. If you should have (close to) 2.5k League Medals and can't wait until you unlock Lab upgrades that take 12 days or longer, Research Potions are also a viable option while the Hammer Jam event is going on.

3.3.3. If, for example, you only ever attack with a very specific army and don't want to upgrade the rest of your lab, Builder Potions are the best choice while the Hammer Jam event is going on.

3.3.4. If you prioritize having all your Heroes available all the time, Hammers of Heroes might actually be your preferred choice, even though their value is really bad. However, if you're okay with your heroes sleeping for these few days while the Hammer Jam event is going on, you can upgrade up to 4 Heroes at the same time and increase their level at an very high speed with Builder Potions!

4. Goblin Workers in the Hammer Jam event

4.1. In general, the Goblin Workers are one of the best ways to spend your Gems in order to progress quickly. In the Hammer Jam event, they are even better than usual: Because of the 50% time reduction, they can complete the work of 2 days in just 1, while still only wanting 40 Gems in return. Therefore, if you ever want to use your Gems for Goblin Workers, now is the time! And if you have the necessary Gems, combining them with Research or Builder Potions makes them even stronger, of course. But also more expensive overall over the course of the 17 days of Hammer Jam.

4.2. Because the Lab upgrades take significantly longer than the Builder upgrades, overall, the Goblin Researcher is the better choice, if you can't (or don't want to) use both; at least if your goal is to max out all of your village (Lab AND Builder upgrades) as quickly as possible. But as explained above, priorities might differ from player to player, of course.

5. Some last thoughts

5.1. Use all the Research and Builder Potions that you currently own or can get through other sources (like the Raid Medal shop) in the event! The 50% time reduction is unbeatable.

5.2. It doesn't matter, which upgrades you do while the Hammer Jam event is going on, long or short. The efficiency is the same. Very long upgrades can be saved up to be upgraded with Magic Items though.

5.3. However, to optimize your time savings you should try to start very long upgrades at the end of the event. That way the time reduction will take effect when the building or lab upgrade is started and "continues" beyond the end of the event.

5.4. Be aware that heavy time reductions are to be expected shortly after the Hammer Jam event once Town Hall 17 is released. They most likely won't go up to 50% or so, but they can mess up your estimates. Example: Assume Lv18 Archer Towers (usually take 3 days, 1.5 days in the event) are reduced to only take 2.5 days in a few weeks with an upcoming update. In hindsight, the reduction to 1.5 days only was -40% in time, not -50% anymore. This is still a lot, of course. Just keep it in mind though :)