For me, the Captivate is the clear choice... here's why...
Captivate runs at 1ghz, which is almost twice as fast as the Aria (runs at 600mhz).
Captivate has a 4" screen, the Aria is only 3.2"
Captivate can capture HD video footage, the Aria cannot.
Captivate comes with a 16gb storage card, the Aria only comes with a 2gb.