T-mobile contract questions?

I'm with T-mobile and my 2 year contract is up in Dec. but i really hate my Mytouch 3g phone and really want to get something with a physical keyboard, preferably the Mytouch slide. I have on and off problems with my phone, it sometimes doesn't let me make calls or text back until after i've retried it 10 times as well as not getting all of my calls, I went into the store yesterday to see if it's possible to even exchange like or pay a small fee for a different phone that's comparable to the one i have because im not happy with the phone i have. and i feel they didn't listen to my questions clearly and said i needed to pay 350 if i want to buy the mytouch slide, i don't want to buy it i wanted to know if i can exchange it. The slide is 49.99 with a 2yr contract and when i got my phone it was 149.99 with a 2 yr contract so in my opinion i should at least be able to exchange phones with possibly be paying a small fee for doing so. Tell me what i can do or what because i really hate this phone.