Smart Phone Help

[mooby]

I'm going to copy and paste what I wrote on FB here in regards to my queries for getting a new phone, and if anyone could help that would be awesome:

I need some advice. Planning on getting a new phone sometime in the next couple months, and I'm undecided between Verizon/ATT/Sprint. I currently have the Iphone 4 with an unlimited plan with Verizon, but if I upgrade they're going to make me pay $10 more for a 2gb plan instead of unlimited data. I'm not happy about paying more for less, but I'll manage.

I do have an upgrade credit if I stay with Verizon, but I checked out their website and it didn't change the price for anything (it said 200 for a Galaxy S3 with a new 2 year plan, and the price didn't change even after I applied the credit, so I'm not entirely sure what that's good for). Also I'm torn between waiting for the latest and greatest (i.e. the Sony Xperia Z that probably won't be out for 3-4 months, and not sure what carriers yet, or the next Iphone) or just buying an older model phone that works fine, like an Iphone 5 or the Galaxy S3. Anyways, any of my techie friends wanna weigh in? I've always had Verizon so I'm not sure how much different the main two competitors (ATT or Sprint) are in terms of network, plans, charges, etc. If I do switch it'll be to either of those two.