Right now, I am in the process of dropping Sprint and migrating to T-Mobile. Should I wait for the S2 assuming T-Mobile gets it since Phonearena said it's a national GSM phone or get the Nexus S because I don't know if I can stand Sprint's crap any longer.
I just switched from a Nexus S to the Galaxy S II - not out of necessity but because my contract was up for renewal. The NXS is a very nice, responsive phone. I love almost everything about it, with one big exception: its web browser. The browser's just slow as heck. If you can live with that, you'll probably have a lot of fun with the Nexus S and the timely updates to new versions of Android that come with it.
The Galaxy S II is, beyond any doubt, is the best smartphone on the market right now. It's much much faster than the iPhone 4. I don't intend to bash the iPhone or any other phone here - but it's a simple fact. the SGS II is the phone that every Android user has been waiting for. Namely the lag-free Android handset. EVERYTHING about this phone is fast. It cuts through flash-enabled websites like butter. The browser is probably as close to perfect as any mobile browser will ever get. At the same time, it has extraordinarily good battery life (comparable to the NXS), a really good camera, a fantastic screen and some additional features (like built-in DLNA support - you can literally send videos, photos and music from the phone to your TV without any fuss, KIES Air, which allows you to access your phone's information through any web browser - it also allows you to upload content (like ringtones) to your phone from any browser) that makes this thing a super-phone. The screen is as bright as the NXS's but much larger. However, the phone is very light (about 25 grams lighter than the iPhone 4), making it highly pocketable.
Anyway - as it's not available in the US and it's unclear whether the "pure" SGS II will ever be (or whether there's going to be a large variety of SGS II-derivations for all kinds of carriers, each with individual feature sets), it depends on whether you want a new phone or whether you need one. If you need one: go with the Nexus S - it's a great phone. If you just want one, wait for the SGS II.