I've been resisting a smart phone for a LONG time. I've had tablets - Nokia 770, 800, now a Nook Color w/ CM7, etc. but I've never wanted a phone to be much more than a phone.
Obviously, I would say get the Aria. Stock they both have (I think) a 600Mhz processor. The 3gs might have better resolution but I think the resolution is fine on the Aria. I do have to say that I have compared my phone with my sisters' 3gs and mine feels faster. I do know that she updated the os and had nothing but problems and had to go back to whatever it was. Ha Ha!!! Anyway, I don't want to get started on the black turtle-neck wearing nazi!