Glad to see this thread. I have been pondering the exact same question. I feel like my Droid 3 was obsolete the day I got it (a little over a year ago) because phones were already going to 1 GB of memory and the D3 was stuck with 512 MB. At the time, I didn't think that would matter much on a phone, but as apps have gotten more sophisticated and I've become more reliant on apps that always have running processes (like Tasker and Locale), the RAM is getting more and more crowded. On top of that, I can't bear to use my all-time favorite keyboard (Swiftkey) because it is such a memory hog, often gobbling up 30+ MB all by itself.
So, I'm going to have to make the jump to a shiny new device or risk losing my mind waiting for my D3 to catch up to what I'm trying to make it do. I'm very tempted by the GNexus for all the reasons listed above: stock Android, quick availability of future Android releases (including Jelly Bean) and easy rootability/hackability. On the other hand, I like that the S3 has a whopping 2 GB of memory, a sporty CPU and GPU and I also like the general shape of the physical hardware compared to the GNexus. I'm not really crazy about having TouchWiz instead of stock Android, but I've been able to put up with (read: avoid) the Moto-Blur "enhancements" on my D3 and, of course, the S3 is now rootable.
I think I've pretty well made up my mind that I'm going with the S3, primarily because it is the most future-proof device available right now. Oh, and the I'm glad to see someone finally put a stock battery on a phone that is able to deliver some usage time between charges. I haven't quite pulled the trigger on the purchase yet, but I think it will happen within the next couple of months at the most.
-SR-