I would the S3 is the better device by far, though we won't know for sure until its in hand. I've had a Razr since early this year and I'm already getting sick of it. The phone looks and feels awesome, and the reception is great, but it leaves alot to be desired. The screen and camera are decent, but far from great, my Charge blew it out of the water in those areas, as well as in sound quality through the headphone jack.
My main issue with the Razr though is the frustration that a locked bootloader causes. Not only can we not make custom kernels, it makes flashing new builds much more difficult as well. VZW and Moto made it sound like the Razr would be one of the first phones upgraded to ICS, not long after it came out. It's now in its 7 month, and still they haven't even given a date for ICS. There are a few options out there, but none of them are great. You can run one of the AOSP builds on top of the stock GB kernel, which gives decent performance and great battery life, but lacks many essential features, like working hardware acceleration and video capture. There are a number of leaked ICS builds, but they're tricky. With Moto you've got to go in order for FW updates, so if you want to revert back to stock you've got to fastboot to the original FW version, then take each OTA in turn to get the next newest version until you have the latest. Well only the 2 earliest ICS builds will allow you to fastboot back to the original firmware and stay on the upgrade path, and those builds get poor battery life and aren't as snappy as they should be. There are many newer ICS builds with better performance, but they do not allow you to fastbook back to GB, so you're stuck with that version. Each one is designed to update from the latest official OTA, and the actual ICS OTA will be the same. So when it finally does drop, everyone on one of the newer leaks will likely be stuck there. A solution may be found, but it will be even more complicated than what I already described.
Compare this to Samsung, where you fire up Odin and flash whatever the hell you want over whatever you currently have, and don't have to go in any specific order. If all that stuff I described in the earlier paragraph confuses you, it confuses us Razr owners as well and is frankly a huge PITA. So while people complain about Samsung updates, Moto is absolutely horrid, as its FAR more frustrating to wait for an OTA. The hardware itself is great aside from a few things, though if you're big on having a great screen you'll be a bit disappointed.
Consider this as well, as far as updates. The SGS3 is Samsung's flagship device, and I'd expect they'll do a good job of keeping it updated, even as Jellybean comes out. Moto, on the other hand, still hasn't updated the Razr while its been their flagship for more than half a year, and it'll likely go EOL (end of life) as soon as the Razr HD releases. So that alone should be enough to tell you that you'll have a far better experience with a Samsung flagship than a Moto one.