Hello!
I first started to play this game here on EN server at the time of OBT start. After premium shop release I got somehow disappointed and gone to try playing on JP server.
And here is my summary of JP server vs here.
Plese, consider this topic as created for INFO purposes and all of the conclusions are IMHO.
1. On JP server it's easier to level up. I just followed the quests and I'm already in the second town without any additional runs & have 15 lvl. When I played here, I made at less 4x Maniac pre-last section & 8x Maniac Best Showtime to leave the first town with 15 lvl.
2. On JP server the drop rates are better. This true for crafting material and the gear too. Also, bonus reward after instance which you unlocks with the key comes in more quantity.
3. On JP server you need 9x less crafting materials for crafting fashion (maybe another stuff too, haven't checked yet).
4. On JP server login bonus table is MUCH-MUCH better.
5. On JP server you have 3 resurrections per day for free.
6. On JP server you can store up to bonus 600 energy (here is 400). (not confirmed yet)
7. On JP server premium shop prices in near 8x less. (Gameforge WTF?). Also, the shop is built-in with preview, not just web-page.
8. There are no XP\Money books and so on in the premium shop on JP server.
9. Login bonus counts separately for every character on JP server.
The game version on JP is more up to date, but this isn't really a thing to mention here at the OBT phase.
The thing that is worse on JP is ping (it depends on your connection, but it can be unplayable for you).
As mentioned by someone, main chat language is JP, so this is also a "-" for some of us.
If you liked the game and don't like GF's approach, I recommend you to try playing to JP server.
UPDATE 2018.3.11:
Today I played on GF servers again.
1. GF increased drop for healing kits (and looks like by a lot).
2. At the moment online is WAY better on GF servers (can't say about endgame). You can sell & buy gear, AH for low-level stuff is pretty active as everyone is leveling.
3. You should be fortunate to find a player using auto team while leveling on JP.
4. Overall, GF EN server feels funnier to play because of many active players currently.
6. Ping, for me it doesn't really matter, caught a lag on JP server only a few times, but it entirely personal and depends on your location & ISP.
TL;DR: GF, please take some inspiration in JP server and make our experience better as your players, thanks.