New Phone Suggestions

Hello,

So, I need a new phone. Here is what I want in the phone:

1. Unlocked (so can be used anywhere in the world)
2. 4G
3. Price not to go over $700
4. No iOS or Blackberry. Android and Windows Phone are a go.
5. Need to use the phone on prepaid.

Bring on the suggestions. Also, by the above 5, I assume, I will only be able to use it on T-Mobile right? I'm in NYC right now.
Also, by a phone now or should I wait a week? maximum I can wait is a week.

Thanks,
Crazysah

PS - I was told I can't use the Samsung Galaxy S3 on T-Mobile prepaid with 4G cause it uses a micro-sim?