This is how I personally decided to choose between the new S3 and Nexus, the S3 has;
-microSD
-better camera (8v5)
-bigger battery (2100 vs 1750, although latter has extended battery)
-slightly larger screen (4.8 v 4.65)
-faster cpu and gpu
-(eventually it will get CM9 and AOPK as well)
Nexus
-curved body, less ugly (opinion)
-better softwares (stock + updates)
-pentaband
-$340 cheaper (S3 16gb released price $739 VAT included vs Nexus $400, used low as $350)
-(USB OTG can alleviate the needs for microSD)
So choosing between the two weights on whether I want to pay the $340 difference for the hardware improvement, my decision is no. I can live with lower megapixels, 4.65" is fine, S3 hardware is faster but practically Nexus can do all the tasks. So I ended up ordering the Nexus. However, I'm waiting to see how much the S3 prices drop by Christmas. I predict since its a part of a popular flagship series and quadcore is still new, the price won't drop as fast as the Nexus did, which nearly half its price in 6 months. I will more likely wait for the next Nexus device if its announced by then.