LS6 is 346 ci
LS2 is 364 ci

LS6 has a 75 mm TB and LS6 intake
Ls2 has a 90 mm TB and an all new intake that should be VERY good

LS2 has a slight bump in compression over the LS6
LS2 has a few sensors located differently

Both use LS6 heads except I believe the LS2 does not use a soduim filled hollow exhaust valve..too expensive to put in a high volume production motor...the LS2 will be in trucks, GTO and several other vehicles.

I THINK the LS6 cam is probably a bit more aggressive...just slightly.

The LS6 HP graph does peak higher than the LS2 but the LS2 has more HP under the peak, or under the curve. I think the LS2 may actually be as fast or faster than the LS6 in the same car.

Other than that they share most of the same dimensions and parts should be relatively interchangable with LS1/LQ4/LS6/LS2

Having the 4" bore aluminum block is going to be a godsend. Add a 4" stroke crank and youll have a 402 ci aluminum motor with factory sleeves. Naturally aspirated 500+ rwhp 402 aluminum motors may be a common sight in the next few years.