i have had both. in fact i just switched to tmobile from cingular, and use the same unlocked phone (nokia 6230b).
i switched mostly to take advantage of a deal (got a moto slvr from tmobile for $75 for switching which i ebayed for $300 so basically got paid $225 to switch).
but i have discovered some advantages to tmobile.
tmobile customer service is like 5000 times better than cingular.
i actually have SBC dsl and chose the combined bill option with cingular (which they now own) . anyhow, for example... call sbc and if you get through you get an indian person.
that already sucks.
the other thing is if you call phone support for billing on cingular and are on combined bills, you must call during norma lbusiness hours.
contrast that to tmobile who i had to call the other day. i called at 1am. talked with a very friendly english speaking american, who was very helpful about a billing question that i was confused with. they even flagged my account to call me back in 5 days to make sure the problem was not a real problem.
i mean for the same cost, usually the tmobile phone deals are better as they give more incentive to the stores that sell their phones (thus an unlocked slvr euro edition is $75 there ) AND they have better customer service and more rate plan options.
i mean for $39.99 you can get 600 minutes + night/weekends. or 1000 minutes. honestly tmobile > cingular.
one thing though is that cingular has better branded phones. i normally get my phones from little mom and pops stores that will sell you unlocked international phones (which they pay for with the cingular / tmobile signup fees they get) , but if you go to a real cingular or tmobile store , cingular has much better phones (i.e. my 6230b which was a cingulra phone, and their current slvr etc) . then again you will get raped at the official stores so dont go there. if you watn a deal on an official branded phone use amazon.com
honestly i just switch back and forth every year (i keep my phone 2 years generally, so basically i get $200 or so every other year for switching), with unlocked phones to get free phones but the years i have tmobile its the little things that make them better. reception is identical here in socal so that isnt a big factor.