My problem with the 3.3 is most all of them Ive had, had rear bearing failure.
Aside from that, the Losi 3.4 simply makes more power. My HPI Bullet currently has my only 3.3 I have left (with Avid ceramic bearings). My brother's HPI Bullet has a Losi 3.4. I simply can't keep up. Top end, they are very similar. The 3.4 has measurably more bottom end, however. The 3.3 isn't a "bad" engine by any means. I just prefer the 3.4 in every way. Aside from being extremely hard to find now..