[Epic 4G General] bitbang3r:
Another thing to consider, especially since you're in the midwest: WiMax might or might not have good indoor coverage today, but EVDO has pretty reliable coverage just about everywhere I've been (including Ohio and Pennsylvania). I'd seriously recommend asking some T-Mobile users who live in areas where you go a few times a year (parents, vacation spots, lakeside resorts, campgrounds, etc) how their 3G coverage is THERE. From what I've seen, T-Mobile has decent 3G coverage in Florida and California, but the more miles between you and the Atlantic or Pacific, the less likely you are to get reliable UMTS 3G from them. In many parts of the midwest, you'll be lucky to get EDGE. In quite a few shockingly non-rural areas, all you'll get is GPRS.

Don't just look at official "coverage" maps... get the Sensorly map viewer (which uses crowdsourcing to have users voluntarily sample signal strength in various areas and report it to them) and check the 3G coverage in specific places where you're likely to spend time (including malls, shopping centers, homes of friends and family members, etc). Sprint is far from perfect, but T-Mobile's 3G coverage looks horrifyingly ragged once you start zooming in to specific neighborhoods, regardless of how good it might look when you're zoomed out so a city the size of Chicago fills a 320x480 screen.

It's been years since Sprint has even *had* meaningful service as slow as GPRS (as in, Samsung SPH-i300 circa 2001-2002), but I can tell you... GPRS is about as close to useless as you can get. It's almost worse than no service at all, because at least when there's no service at all, you won't waste your time bothering to even try using it.

GPS wise, a stock Vibrant will never be more than a week or two ahead of a rooted Epic -- the moment any real fix gets released for the Vibrant, it'll get ripped and grafted onto Epics anyway. The same holds true in the other direction as well. How many hours did it take for Epic's GPS improvements to make their way onto rooted Vibrants? Like, maybe... 6?

The 150k EVDO cap is a bug, and I suspect the only reason it hasn't been fixed YET is because it probably requires taking a cell site down for maintenance, so Sprint is loath to do it outside of a regularly-scheduled maintenance window. Whatever phone you go with is going to be with you for at least another 10 months... do you really want to commit yourself to 9 of them based on a problem that's likely to resolve itself by the end of next month?

Also, don't forget that the Vibrant has only a small amount of "fast" flash, and relies on an internal microSD card for the other 16gb. On an Android phone, the only place slower than MicroSD for storage is a remote server accessed over the 'net.

In short, don't cut off your nose to spite your face... at least, not without spending some serious time researching what T-Mobile is like in the specific square blocks of land you care about. T-Mobile's best areas might be better than Sprint, but it's a lot worse than Sprint in quite a few more (especially the middle parts of the country, and more than a mile away from a transcontinental interstate highway). You'll be able to make voice calls & do text messages just about everywhere, thanks to roaming agreements with AT&T... but AFAIK, you can't roam on 3G, and I'm pretty sure AT&T won't allow you to roam on EDGE either. In contrast, Sprint can roam on Verizon and enjoy at least 1xRTT speeds anywhere Verizon has service. 1xRTT isn't fast, but there's a blurry line somewhere between "annoyingly slow" and "unusable" -- 1xRTT is somewhere on the good side, and GPRS is absolutely on the bad side of it.