the size is pretty similar, my buddy was comparing his hero the other day. the screen is the same and the height/width is similar. the biggest difference is thinkness, the Aria is about 1/2 the thickness.
the other difference is speed, my buddy runs 2.1 and it is super slow so he liked how fast the aria was. the big difference for him was the physical keys. he liked that any of the keys would wake the phone, were on the aria it is the power at the top. however, how i grab/hold the phone the power is right there for me and i don't think that's as big a deal. i don't miss the physical keys of my Treo pro.