You can't really generalise
From what I gather, depending on which part of the US you're in different networks are at different stages of 'build out'.
So, in some areas you'll actually get great Verizon coverage, and in others you'll get excellent AT&T (Cingular) or even T-Mobile coverage.
So, really, I'd suggest you get local advice (and not from a sales person) as to which network's best for you.
Also, remember that AT&T and T-Mobile use GSM (and will use UMTS 3G) technology while Sprint and Verizon use Qualcomm CDMA.
GSM networks will generally have access to a *MUCH* better range of handsets as GSM networks have nearly 2.5billion subscribers worldwide so it's a vast market and has a vast array of handsets.
The US GSM networks are years behind on 3G though. In Europe, almost all of the networks are well on the way to full 3GSM (UMTS) roll out, so you can do things like access the internet at 3.6 - 7.2 Mbit/s from anywhere, make video calls, browse the web at genuinely high speed on your handset all that kind of stuff.
AT&T is still mostly stuck with EDGE for the time being, a kind of half-way house technology that's basically GPRS (basic mobile data on GSM) on rocket skates. It's not 3G and no where near as fast though.
Your 3GSM rollouts and other 3G rollouts on CDMA networks have been substantially delayed because the FCC didn't allocate the necessary frequencies.
In Europe :
GSM works on 900mhz and 1800mhz (2G/2.5G)
and for 3GSM (UMTS) 1900Mhz is used for uplink and 2100Mhz for downlink.
(As 2G GSM becomes less popular, the 900Mhz spectrum will also be partially used for 3G services)
In the USA the FCC tried to squish EVERYTHING into 1900Mhz and 850Mhz i.e. sharing bandwidth with 2G phones (GSM/CDMA) and wouldn't licence further bandwidth for 3G. This resulted in very bandwidth-limited 3G services and drastically hindered the rollout.
What's happened now is that the FCC has cleared some of the 2100Mhz spectrum (used for very old satellite stuff in the US)
So, it's hoped that operators will be able to use 1900Mhz (Shared) for uplink and 2100Mhz for downlink which means that you'll have all the European and Japanese handsets available.
As it stands, each US network operator's 3G setup is different. They're big networks, but no where near as big as the market that simply uses European standards i.e. the rest of the world. So, you've a few million subscribers versus potentially 2.5billion subs.
So - basically, if shopping for a mobile phone in the US, understand the technology, understand the technical limitations and seek good local advice on which network provides the best coverage in the areas you're likely to be in.