Fork, for obvious reasons.
Wheels, need a good set of wheels that won't go out of true after each ride.
Tires, got to be tires in which the knobs won't fall off on the first accidental skid.
Feel-which ever bike feels best to you, that is the bike to get, if it is the one that has the lower parts, get that one then use the money saved on upgrades as the parts break.