I am in the same boat. Ordered the S3 and currently rocking a Rooted Nexus. Sigh, I feel like Google just through a wrench into my plans lol.
I saw the GS3 in person and I love the device. Screen is definitely nicer than the Nexus, Screen is bigger in size however the phones are almost the same exact size. S3 is slightly (ever so) bigger than the Nexus when placed on top of it however it is noticeably thinner.
Touchwiz wasn't horrible, in fact it had a slew of great features, however it did have its negatives. It felt like a great ROM but a little bit cartoony for my tastes. Also coming from stock ICS I was a little spoiled by the Stock Keyboard (Nexus > S3) and the folder creation (Nexus > S3). I did a keyboard test and on both phones I typed "heybthere" (without the quotes). My Nexus changed it to "hey there" and the S3 decided I wanted to write "Heather". Imagine if I were writing
hey there, whats up? to my wife and she receives
Heather, whats up?, lol. As far as the folders, on the Nexus you just drag one icon over the next, however on Touchwiz you need to press menu, create folder and then drop things into the folder (lame). Also why is there a dedicated menu button, I thought I attended that funeral already.
The S3 definitely had plusses however, like the Motion controls, not for everyone but I liked them and they worked well for me, all of the Power settings in Notifications bar, (yeah I know a ROM can do that for the Nexus but I stay stock rooted), Silky smooth scrolling everywhere (I'm looking at you widget drawer) even with the Live Wallpaper on. There were plenty of other goodies as well.
I dont know. On one hand the S3 is much more future proof as far as hardware goes, however we are going to get stuck waiting for updates (which will drive me mad). 2 gigs of RAM and touchwiz or 1 gig and JB. I feel like I am talking myself into keeping the Nexus but then I think about the camera, oh and the possibility of having superior connectivity in places that the Nexus 'takes a break' from the Google servers.
If I had confirmation from the future that there will be a Jelly Bean ROM for the S3 quickly then that would be my first ROM.
But then again maybe this time I can actually buy some official accessories for less than $90 considering it will have the same form factor over all 4 US brands. That would be nice.
Sorry for the flippity floppity book like posting. Just thinking to myself and typing to whoever will read.