I do have a question for you though. You said they've offered a Gnex for free, or SIII for 99 bucks with renewal. I assume you mean that the GNex would ALSO be a renewal situation? I've never heard of them offering a "free" replacement under warranty for water damage. That would be an insurance claim if you have it, and would cost you $100 anyway. That said...
Seeing as the SIII is $99 online anyway, it's not like they're doing you any favors by offering that price to you. And the S4 is only $199, so if you HAVE to upgrade, I'd try to come up with the extra and go that way if you can. But like jmar said, if you have the older unlimited data plan, doing any renewal, regardless of phone model, is going to change that. So, depending on how much data you use, that could come into play. If you're already on tiered, then I'd say try to hold out for at least the S4, if you can. If not, might as well go with the SIII since it's a tad above the GNex as an all-around phone. I love my Nexus, wouldn't give it up for anything out yet (Full price for me, yuck), but I wouldn't lock in another contract term renewing on a phone that's already been out for that long.