Regardless of choice it seem to me like you'll be happy with either choice you make. Since you have experience with both OSs so you know their strength and pitfalls.
Unlocked phones are expensive get ready to pay full price for a phone. The best deal right know is the Galaxy Nexus at $349 it's still considered top of the line and has the best value new. Everything else goes for about $600 and up unless you buy a low end phone or a used phone. There is a tmobile version that supports 4G
One other thing to consider,if you get an iphone is to switch carrier since the best Internet speeds you'll get is 2G for unlocked iphone on tmobile.