If you're already using t-mobile, and you're happy with the service, stick with them.

Verizon may have better coverage in the USA, but a future Nexus Verizon edition may be locked into Verizon - you will no longer have a choice of carriers. If you don't plan to move or travel overseas, it's worth investigating.

I personally am using AT&T prepaid. I am near wifi most of the time, and don't use the phone much. Cheap.