Is the 9v typ input voltage would be within the acceptable range of both?
9V is acceptable for neither chip or indeed any AVR, however if you're talking about VIN to an Arduino board with a regulator that that's OK.
get a mega. I guess the question is...
...Is this ambitious or overkill
It may be overkill for many projects, but then again not for others. It's no harder to program with a Mega than a standard Arduino so "ambitious" doesn't really come into it. In fact it's probably easier to program the Mega because you have more resouces.