The CrowMags can be devestating at 20-25yds but at 30 the groups start opening up. The Preds are better for longer distances but you need to shoot whichever is most accurate in your gun. Like Jim said, at 600-650fps, either pellet will not expand much. You need higher velocities to get the full effect of a hollow-point pellet. I am not sure if the 6" groups with the CM's are due to the bore being dirty. That is just too large of a group. Your gun most likely does not like CM's, and it won't shoot much better after you clean the bore. CM's typically like 700-850fps velocities a lot better.
Sorry you haven't had good luck with the Crow Mags. I'd clean my A/G thoroughly before I made a final decision on any pellets. I forgot what you're using, but I think it was a Crosman pumper....yeah, I'd be shopping for a hunter A/G. That Crosman is a fine plinker and will love the cheap CPHPs from Wally's. Hang onto them and use them in your next A/G. They are a great hunter out to 25 yards.