Nexus 4 answers needed.

As of right now I have a iPhone 4S and have been a iPhone user since the first one was released(I did skip the 3G for the G1 when it was first released but switched back to iPhone because of how terribly slow the phone was). I am now bored out my mind w/ iOS and plan to switch to Android. Like every other person looking to upgrade their phone, I have my eye on the Nexus 4 and SGS4 but am leaning towards the N4. I would like LTE support and expandable storage but not if it costs me updates when they're released. Any input on this would be appreciated.

On to my N4 questions. I have been reading that you can get the N4 on T-Mobile for $30 a month, what all comes in that plan? If I use the N4 on AT&T(I'm grandfathered into unlimited Internet until I change my plan) will my N4 have the same HSPA+ speeds as it would on T-Mo?