The Ativ is lot lighter. I currently have a GS3 and while I would never call it an attractive phone, I do enjoy how light and thin it is and don't think it's "cheap looking". Either way the size for me isn't a deal breaker. Most people (not me though) put cases on their phone so you'll be bulking up the Ativ a little bit where it looks like maybe the 920 won't need a case?
The Ativ also has a removable storage card which some people will have as a requirement because they like to carry all kinds of data with them. For me it's a non-issue, but it will be a deal breaker for some.
Initially it looks like the 920 Camera will dominate all other phone cameras so a big +1 to the 920. I love taking random pictures and I refuse to carry a seperate camera with me. But that's just me, YMMV. And to be honest.. the Ativ camera will be pretty good too, assuming its the same as the GS3.
Radios will be a big one. The radio/signal strength in the VZW Galaxy Crapus, er Nexus, sucked. The GS3 is ok, but still not stellar. Folks in larger cities with towers all over the place and strong signal didn't seem to ever have issues, but if you were in places with just medium signal... you'd suffer. I don't know how good Nokia will be, but I'm going to start off assuming the Ativ will be as good as the GS3 is. Which I can live with.
I need free Turn by Turn Navigation, but I think that will be the same between them both.
I love the multiple color choices of the 920. My phones always go naked so I can let those colors be seen.

I'll probably end up with grey though...haha.
The GS3 design is bland and boring, but from the screenshots of the Ativ is seems more "exciting looking" then the GS3 so I'm ok with that too...
They are both running WinPho 8, so a big **** yeah for that!
So, in conclusion, the only thing that matters is which bad boy comes to Verizon first.. If we can pretend and say by some miracle they both launch together, I would have to go for the 920. The main reason is that it seems more "exotic" because I've never rocked a Nokia smartphone before and I've had tons of Sammys all the way back to the i730 WinMo bad boy. All I care about is WP8 and taking pictures so either way I think I'm going to be happy.