Coverage for me is key and my legacy unlimited data lines. VZW wins both where I live and the areas I travel. An apparent trump though for folks that are indifferent about coverage is At&t has stated they intend on carrying the 32Gb version, but VZW are mum about it.
Either case, a bigger option is more likely June-ish probably for any carrier. That said, if you want something now and storage is not an issue for you (as so for many), you kind of have one option of two right now. At&t stores have 16gb in stock. I stopped messing with the demo, since makes me look at my S3 in a less flattering light.