Well the HTC aria has better specs for the CPU and memory, a multi-touch screen (as opposed to a regular touch screen) & FM Radio (which the backflip does not)
the Backflip has Stero mic and loud speaker (Aria has mono on both). Also has a QWERTY keyboard while everything is done of the multi-touch screen on the Aria.
Other than those few differences the phones look to be the same. Overall I think I woudl chose the HTC Aria between the two.
oh I didn't really look at the size of them :P. Just the technical specs:
I'm getting a contract update on AT&T soon, and im trying to decide between the HTC aria and the Motorola backflip.
yeah, i'm beggining to lean towards the aria, but the size is an issue