The #1 biggest item is reliability. I've shot side by side with a friend with an SLP for the past 2 years while initially I shot a 930SPX. The SLP has been very reliable, almost perfect while I wore out my 930. I just bought an M4 Tactical last week and we went out to shoot a Tactical Shotgun course, 400 rounds. Now both of our guns were near perfect. I like the self cleaning ARGO piston system better and the locking bolt on the M4 seems like a good idea although I haven't seen an accuracy advantage between the two. The takedown of the M4 is right out of the SOCOM requirements which is definately the fastest and easiest out there. Both come with rails and quality ghost ring sights. A FN Police SLP runs around $1025, while a standard M4 would have run me $1325 so for $300 more there are a few features that come with the higher price. Overall both are great guns and I wouldn't hesitate 1 second on getting an FN.