GPS is more accurate for speed...otherwise speed will be calculated based on RPM,gearing,battery,tires,etc.The telemetry speed is a bit optimistic. I just use the telemetry for speed...it's not worth another $100 to me for GPS speed. Whichever works best for you.
Slash Ult. HW WP SC8/2150kv MIP King Headz
Rustler 3500kv VXL