Doubt i'll sell my nexus as I never sell old phones, but I will probably get a GS3 at some point, as it should blow Nexus off the planet.
As for software updates, GS2 gave more updates, at a much faster rate than Nexus, so GS3 should be much much better for updates.
Nexus has Android 4. but I doubt it will get 5, and the speed of the incremental android 4 updates is a joke.. 1 update since I got my phone.. GS2 had updates all the time! I'd rather get incremental updates within Android 4 timely with an SGS3 than waiting an eternity for any update on Nexus.
Nah sod the notion of speedy updates on nexus devices, they don't exist.
I'd take Touchwiz and a fast Galaxy phone over the slow and laggy Nexus any day. GS3 will hopefully have equal screen than nexus, which is the nexus's only real selling point.
It'll be nice to have a phone with a HD screen which you won't have to hear all the excuses that it is so slow because it has so many more pixels to push blah blah blah.