Honestly, I'd rather want a 3 liter H4 instead of a 3 liter H6. Then again, that would be a simply massive engine.
A 6 cylinder 2.5l engine should have about the same horsepower output of a 4 cylinder 2.5l engine, but the 4 cylinder should have more torque. This is assuming that the stroke is simlar. Then again, you would come upon some other problems, such as having a massive amount of mass due to a huge bore.
As for transmission, if you really want performance, best to go with RWD over AWD. If you are planning on racing in snow or low grip situations, AWD woudl be the way to go.