both do have their advantages and disadvantages. It also depends on your draft position and the average money invested in the scouting process in your league. I personnally do prefer the scouting combine; if it shows me at least 10 players with 1 or 2 grey balls who are 18yo, I usually follow it up with the group demonstration.
and while there probably is a little difference between the probability of greatness for a 5 grey ball player and a 1 grey ball player, the effect of this rating is pretty limited, so don't fear using it.
Jus my two cents.