I have both devices. I find that the Arc's display renders the colors for images like contact pics a bit washed-out even at the same brightness level as the O2X. I also find the viewing angles of the O2X better than that of the Arc's. The viewing angles weakness of the Arc's Reality Display is something some professional reviewers have also pointed out.
Now, some people prefer that sort of washed-out look but if a friend asked me which to get -- just on the basis of the display I'd tell him to get the O2X. The colors are just better rendered and warmer/more lush. Some of you may not know it but it's actually a lower-res version of the iPhone 4's IPS display (which LG makes for Apple) - 480 x 800 (O2x) vs. 960 x 640 (iPhone 4).
What's more, those who frequent this site may know that Paul's already got an excellent custom ROM for the O2X that's solved most of its niggles. I wish he'd make one for the Arc.
The Arc has good battery-life whereas the O2X's is bit more average, especially with Wi-Fi on. I wonder if that will improve with better drivers when the O2X's gingerbread update comes but you'd best have a portable charger if you do get it.
The Arc's quite zippy but does have occasional lags -- not anything show-stopping or even annoying, honestly, but I wouldn't put 100% faith in those reviews that said they got absolutely zero lag. Again, it's a matter of preference but the Arc is a great example of a pre-dual core device. I was actually thinking of waiting for the HTC Incredible S, which is another pre-dual core device, but my dealer gave me good trade-in value for a couple of phones I didn't use (an e72 and an N900) so I bit the bullet with arc. I don't know if I should have waited but they're more similar than not in terms of specs: http://www.gsmarena....p;idPhone2=3788
I guess the Incredible S wins out on the more generous storage and better mobile network connectivity speeds but I'm sort of getting tired of HTC's designs and SenseUI does tend to slow a device down.
An annoying things with the Arc, though: it ships with some pre-installed software like Postcard, TrackID, Play Now and Office Suite (I prefer DTG or Quickoffice) which eat up valuable device storage. When the custom ROMs are out, not having those (and the other pre-installs) loaded could save anywhere from 25 to 50MB, which is a lot since it only has 512MB of storage (about 320 available to the user, though out of the box, mine had about 260MB). In contrast, the O2X has a ton of space; I've installed quite few programs and I've still got 1.2GB left.