HSPA+ will be available in a larger market than LTE for probably several years or more. The speeds are not as fast as the fastest LTE areas, but are probably on par with the average LTE speeds. You can get >20Mbps over HSPA+ 42, and it likely won't use as much battery while doing it, as LTE requires a separate voice connection.
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If you have to have the fastest connection, maybe you'll want to wait for LTE. But there are still plenty of people on HSPA 14 or 21 devices that get by just fine. For comparison I believe the iPhone 4 is HSDPA 5.6, while the 4s is 14. I think HSPA+42 would be acceptable for a while.
There are a lot more variables involved, making it difficult to provide answers that fit everyone.
For example, when I got my T-Mobile SGS2 it was one of only two phones that could take advantage of T-Mobile's HSPA+42, which they had just rolled out in my city. At approximately the same time Verizon turned the switch on their LTE here. I regularly beat my friends with Verizon LTE phones in download speeds. Why? Because HSPA+42 is theoretically not quite as fast as Verizon's LTE, but there were so many more Verizon LTE phones out there sucking down the bandwidth that I actually had better speed than they did. It is now a year and a half later and my Verizon friends and I are now closer to par, and both of us have lost a bit of speed.
Another factor is how far your primary areas (home, work, school) are from towers. If the competition has better coverage in your critical areas, then they will be a better choice for you. And scale this up for those who travel a lot - what is the coverage like in the cities you go to?
Finally, my (admittedly dim) understanding is that T-Mobile's LTE will be a second generation LTE, that will be quite a bit faster than the LTE offered by Verizon, AT&T and Sprint. But, even if I am right about that, T-Mobile has made no advance notice as to which cities will get it first, nor is there much of a timetable other than "starting in 2013." So again, it is kind of a crapshoot.
My suggestion is to compare with friends in the areas where you need coverage, that is, friends on all carriers. And badger T-Mobile offices for as much information as they will divulge.