Yeah, it totally depends on what you're writing. There's a lot to consider:
- A 4s is much better to test on, because the a5 makes it much quicker (quicker install, quicker app start, quicker in-app for testing). That saves you a lot of time when you're testing on the device.
- A 4s will hide performance issues. If your code is too slow somewhere, it might run fine on the 4s but dog slow on a 3g. The end result is that you end up with a bad app and lots of complaints.
- Maybe you do 99% of your testing in the simulator and the whole question is irrelevant (I don't have this luxury

)
- Both 4 and 4s are retina devices. Do you need to test on a non-retina display?
- Do you need to test on older iOS versions? That makes things tricky! How about beta iOS versions?
Ideally you need a testing suite, with all the major iPhone and iPod versions, running a range of OS versions, but I guess you need to compromise a little

Personally I have a 4s for main dev work, and a 3gs for compatibility testing (none of my recent work will run on 3g or older, so that's enough).