Quick background: I have been in/out of the mobile phone industry for the past few years and have had service contracts with all the big three.

As far as nationwide CALL coverage (baring small cowpoke towns):

#1 - Verizon
#2 - AT&T
#3 - Sprint
#4 - TMobile

As far as nationwide INTERNET 3G coverage:

#1 - Verizon
#2 - AT&T
#3 - Sprint
#4 - TMobile

Note: the reason AT&T experiences problems in NYC, SF, and Chicago is because of bandwidth issues due to some many folks owning an iPhone. They expect to have this fully ironed out by late 2010. By that time, and also early 2011 AT&T and Sprint will have released their LTE/WiMax/4G/What-have-you coverage and all of them will be pretty good.

As far as TYPE OF PHONE:

#1 - AT&T offers the most flexibility
#2 - TMobile
#2 - Verizon
#3 - Sprint

AT&T and TMobile use SIM cards and thus you can buy any "unlocked" phone and use it on their network. Also, you can take the phone internationally and buy a local SIM card which is usually much cheaper that using your SIM card. It's a bit more difficult with Verizon and Sprint since they do not use SIM cards.

So...in my opinion, regardless of the iPhone, if you're a current AT&T subscriber stick with AT&T as they are working very hard to expand their network bandwidth. If you are wanting to switch from Sprint / Verizon, I'd wait a year. But, no matter who you use, all of the big three (and T-mobile) will get much improvement in call coverage, broadband speeds, and phone/hardware choices over the next 12 to 18 months.