It depends upon what type of game it is and your ability to code.
But overall AS3 is much faster than AS2 -- especially when doing complex math. Many types of games require a great deal of math and therefore they would benefit from AS3.
However a very poorly coded game in AS3 could easily be slower than the same kind of game if it were coded with great skill in AS2.
Alright, so overall, if the person knows what they're doing, it's best to work in AS3, right?
I would say so.