If you have programs, or hardware, that won't run on 7 then XP is best for you. If you want to run programs, or have hardware, that work only in 7 then 7 is the best choice. It's horses for courses, and only you can determine which best suits your purposes. Speedwise there is little, if any, difference - my experience is that 7 is slightly faster than XP on the same hardware.
If performance is your only concern, and you have no other reason to want to upgrade, then don't bother. You know that your hardware is fully suported by XP, so why spend more if things are working OK at present?