OP I have 14 Armyteks including all the V3 Preds and DP's. The biggest difference is going to be basically how easy they are to carry. As the manufacturer stated the DP is indeed more compact than the PP. That being said I personally prefer my Pred Pros and do not mind carrying the extra size. I really like the beam profile out of the XP-L HI PP quite a bit. The beam profile out of the DP is not as wide and has artifacts around the edge from the DP's high crenelations. Not saying the DP is a bad light at all I just got the XHP50 DP and its flat out amazing its actually so bright on turbo I almost never use turbo lol.
Yes both lights can be bolted to long guns.