For me, the choice is clear: Verizon. T-Mobile's converge is vastly inferior, and for where I live, it isn't worth it (since T-Mobile is non-existent in this market outside of major cities).
Take a very close look at where you live, and where you travel on the coverage maps.
If you have Verizon and are on Unlimited data via 3G... for the love of God do not switch. lol Stay Grandfathered in and buy a 4G LTE phone outright (not via a plan) and your Unlimited data should carry up to 4G LTE.
Also remember, T-Mobile doesn't have IRL 4G service (LTE) and probably never will. It sounds like a great deal now, but switching might bite you in the ass, especially if your sitting on a Verizon Unlimited data plan. T-Mobile (and AT&T for that matter) run something I like to refer to as "the FAKE-G." Its HSPA+ or what the rest of the non-United States world calls 3.5G service. While AT&T is installing true 4G LTE towers, T-Mobile is simply trying to sell itself off instead of upgrading its network to compete.
Remember, the only real 4G is LTE.
If your on a metered Verizon data plan, it might be worth considering the switch (and shouldn't really change anything if you do). If you got Unlimited data via Verizon, don't even consider it. Wait for a 4G Windows Phone on Verizon and grab that when it hits at full price (even if it cost a pretty penny) and enjoy unlimited true 4G LTE data.
Read this for more information on keeping a Verizon Unlimited data plan with 4G. The only thing is, it doesn't account for the recently announced "truly Unlimited" T-Mobile plans, where they won't throttle your connection after 2GBs for some extra money a month that was just announced.
I'm not trying to sound like some kinda Verizon fanboy, but I want you to know some facts before thinking about switching.