All things being equal, an appropriately fitted RP Project-X shaft should have a flatter ball flight and tighter shot dispersion pattern than it's DG equivalent.
As a general rule, you should choose the RP shaft with the lowest frequency (5.0, 6.0, etc) you can tolerate. Or put another way, once a golfer starts hitting a straight block or more of a fade on a shot that would have been straight otherwise, you know that frequency is too great and you should choose the next lowest shaft frequency. Example:
My irons with...
5.0: lots of pull hooks with some straight shots. Really hard to
block or fade. Also, ball flight too high - ballooning Shaft is
too "whippy"
5.5: still too many hooks with more on line shots. I can fade
the ball if I really try. Ball flight still too high
6.0: Perfect! I can hit any shot when I want to. Just the right
trajectory. The ball gets up fast but doesn't continue to rise
very long. My pitching wedge doesn't fly any higher than my
5-iron
6.5+: More fades and pushes when I know I hit it square. Not
enough "kick" in these shafts. Ball flight too low, especially
with long irons.