From what I have heard about the two is that:
EDGE=full signal maximum speed
and I have heard 2 things for edge:
edge= a lot of people on network, not maximum speed or
edge= can see tower in range but cannot connect using edge(shows this a lot if you have no data plan)
in this case it shows edge but stays in GPRS mode.
GPRS= fast but not as fast as EDGE(not quite sure how GPRS works)