Nokia 6682 / 6681

I currently own a Nokia 6600 with T-mobile. I wanted to upgrade to the Nokia 6681, because I've been told i can't use the 6682 because it uses GSM 850 which is not supported by T-mobile. My question: is there a significant upgrade to go from a Nokia 6600 to the 6681 or should I wait until the N70 comes out? Also, If I get the unbranded 6681 will it be able to have MP3 ringtones? Is the quality of the MP3 player on the 6681 close/comparable to the iPod?? and Finally other than the GSM900/GSM850, is there any other differences between the Nokia 6681 and Nokia 6682?