T-Mobile, LTE and Galaxies

I was just about to sign up for a T-Mobile Monthly 4G prepaid plan ($30 for 100 minutes, unlimited text and 5 GB of data) and buy a Nexus 4 from the Google Play Store, but with T-Mobile's pending announcement on March 26th, I'm wondering if I should wait.

While I'm really into the N4, the rumored LTE version of the Galaxy S III might be a better buy -- unless the N4's LTE antenna is activated at some point. The GS3's camera is a big plus, but I don't know if it's worth sacrificing the N4's universality and ability to jump onto any non-CDMA network.

Would the LTE GS3 be locked into T-Mobile? If so, would I be able to unlock it? I'm assuming I could take it to AT&T, but not Verizon/Sprint.

Any advice would be much appreciated. I've been researching for the past few weeks, but I'm still on an OG Droid, so I'm not exactly up to date on all the new developments.