I would recommend regular GT6, unless GT6U finally took all the upstream changes from me that fix severe Bugs and such.
And whe nit comes to Modpacks I dont like to tell people which Mods to use and not use, unless there is an objective reason why a Mod is problematic, like Optifine letting Mobs spawn where they should not spawn, despite it not even being supposed to mess with Mob Spawns at all in the first damn place, or CoFH Core and Enchiridion lagging like hell on startup and when loading a world (they dont seem to break stuff once the game is up though, so they are still fine-ish, also enchiridion seems to have a fork that fixed that lag issue on startup).
I would just recommend having fun with the Mods you like. 