Well, I don't know much about this game. (Mainly because I don't even have Half Life, yet..) But anyways, I'll post my thoughts..
I remember reading that transformantions were important in the new release, so.. This is how I believe Trunks should be..
Base > SSJ > USSJ > USSJ2.
SSJ + USSJ are both good forms. They both increase speed and power. Let me make something clear. (I don't believe that this has been said yet...)
USSJ2 does NOT increase speed. Infact, it decreases speed and increases power.
..On second thought, USSJ2 might not even be needed..
Anyways, that's basicially my two cents. But I can't say much considering I haven't played this game.. So I don't know how stuff works..