The $70 plan is not unlimited 4G, it is unlimited data with a 5GB cap on 4G service. There currently is no unlimited 4G prepaid plan. T-Mobile coverage in good in Manhattan, Queens & Brooklyn. I do not know about the Bronx or SI. It is sketchy in the burbs. I am a current T-Mobile user but was previously with Verizon for years and found their coverage was always better than anybody else.
I currently have the $60 plan with 2GB of 4G service. I stream Pandora during my commute daily and have yet to exceed the cap. You can change you plan at any time right on their website. BTW, if you choose this route and have your credit card auto billed set it up to bill about 3 days before the 30 day period is up. By default it goes to every 30 days and that can sometimes cause you to lose service for the final day due to timing issues.