You're asking to compare 2 devices both as of yet not announced officially much less released. We can assume a lot more about the Evo4G LTE only because it's assumed to be the OneX. Still, until someone is able to actually buy the thing it's just speculation. We have almost nothing to go on for the SIII, we don't even know if it will be called the SIII.
Personally, I'm sitting on an upgrade waiting for the next Evo. I'm skeptical about battery life and the lack off a removable battery. If I decide that's not an issue, it will almost certainly be my next phone. I've now owned 3 different Samsung phones, the Moment (me), Epic4G(wife) and Epic Touch4g (wife). All have had annoying issues with gps. It took 4 replacements to get a working gps in the moment, the Epics never worked and the ET4G is slow to lock and unreliable as a navigator. I said after the Epic, I would avoid Samsung and went back on that when the ET4G was only $79 on Amazon. The ET4G is a great device, but why can't Samsung get it's gps act together after 4 years?The SIII may trump the OneX spec wise in every way, but I'll probably avoid Samsung on the next round regardless of price.