Under the hood, both phones are basically the same except Sprint's version has an inferior camera to the AT&T Optimus G/Nexus 4 and with the non-Nexus phones, you'll be at the mercy of LG and the carrier to get updates to you.
I ordered a Nexus 4 because it's unlocked and thus able to use inexpensive pre-paid services like Straight Talk ($45/mo. for unlimited talk and text and data, but the last is in quotes) or T-Mobile's $30 unlimited text/data w/100 minutes of talk plans. With my work discount, I'm paying $78.mo. for my Everything 450 plan and TEP, so lopping ~$30/mo. off using Straight Talk AND getting off of Trudge's slow-ass slow unlimited dial-up data network sounds right to me. The savings in airtime in a year will pay for the phone itself and I'm not tied down for two years.
I still like my GS2 a lot am only moving from it because I've simply gotten fed the f@#% up with Trudge's pathetic 3G slows (can't really call it speeds) and utter lack of 4G. (They never finished WiMax and have no plans for LTE that I'm aware of.) I did some speed tests and sites that load in a few seconds on a co-worker's MyTouch 4G (T-Mo) take over a minute on mine. What good is a fast phone on a broken network? It's like having a Ferrari, but unpaved, boulder-strewn roads. What good is an unlimited ocean of data if you can only sip it through a coffee stirrer? What good is good hardware without the latest software? The GS3 is only starting to get Jellybean and it's highly unlikely the GS2 will ever get it without loading a ROM.