Biggest differences between the carriers is the technologies they use and the footprint in your area and the plans they offer.

Cingular uses GSM technology. So if you plan to travel outside the United States, this might be the best bet. Tmobile is the same.

Verizon uses CDMA technology. Its a very good technology and you have roaming capability with Canada and Mexico but not a whole lot else.

The second thing to look at is the coverage in your area. Carriers tend to have very good coverage in some areas (i.e., city) and less in other areas. For example, if you are driving across Texas you'll be fine in the cities but may not have a lot of luck between them. A carrier like Verizon has phones that do both digital (CDMA) and analog (AMPS) and this covers just about 100% of the US. The digital coverage alone is less that that. Fine for most people - but spotty if you are in very rural areas. The key is that it has good strong coverage at your house, work, in-between. I think the major carriers have this covered.

The third is the plans. A lot of this depends on your family as well. Verizon has plans where it is free to call between people on the same carrier. So if my family has Verizon and I can call them anytime for free. Its a great feature if they live out of state. I am sure others will follow in the future.

Cingular has other plans as well where they will rollover the unused minutes to the next month (for a year). So if you don't use a lot, you don't lose them. A nice feature if its important to you.

As for phones. Most of the major brands have similar models they make for each carrier. Nokia and Sony (from Europe) do a lot of phones for Cingular, Motorola does a lot for Verizon. That being said, I have a Motorola on Cingular and they have models with the same features on Verizon. Key features are battery life, size, features (camera, etc), ease of getting accessories, and ease of use (and price).

I like the Sony phones. They've recently come out with a few new ones with nice features including a 1.3megapixel camera - where the pictures turn out decent (most phones have pretty crappy cameras). They have good battery life and are usually easy to use.