The X10 has better hardware than the Aria. Both actually have very similar touch sensors which have issues with multitouch. SE just decided to omit multitouch rather than incorporate a poor quality version of it, but they've since changed their minds about it, hence the addition of multitouch coming.
I personally don't think either phone is a great choice though. The Aria is running very old hardware. It's actually just a tiny step above the original G1, and it's very likely that the Aria will stop receiving update support very soon. In fact, I'd be surprised if it ever got Android 2.2.
The X10 is much more recent hardware, running a platform similar to the Nexus One and HTC Desire. The problem with the X10 is actually Sony Ericsson. They're just too slow to keep the phone up to date, proven by the fact that it's still running 1.6.
If I had to choose between the two, I'd go for the X10. You might want to consider the Captivate though, or maybe leave AT&T. I'll be leaving AT&T in the next few days unless the Nexus S happens to work on AT&T's 3G network (which all signs indicate that it won't).
