That really depends on you. Debranding will free up some memory by getting rid of any software related to the carrier, in your case at&t. It will also remove the MP3 ringtone restriction. However, by debranding your phone, you void the carrier's warranty. YOu still have the manufacturer's warranty, but it probably won't be as easy replacing it if ever the phone gets defective. If it was bought within 30 days, you can replace the defective phone and get a new one..granted that it's not water damaged or the screen isn't cracked. Also, if you bought it through a new activation, you can have a chance to replace it within 1 year. The process with the manufacturer might be a little bit more lengthly, as it must be shipped in for repairs, then diagnosed.
It's really your choice, if you prefer the generic firmware version or the at&t one. Just something to let you know for peace of mind, if you still just want to leave it as it is.