Funny how personal Surefire selection is. I actually prefer the 618, but that may be due to a lack of experience with the newer lights. I picked up a police trade-in 870 without a barrel but with a 618, peep rear sight, WC magazine follower, and a Speedfeed IV-S on it for $269 from Mark at Summit Gun Broker. The rubber exterior of the momentary switch was separated, so I contacted Surefire for an RMA. It was repaired at zero cost, and I picked up a Malkoff MDSFFE-2 LED upgrade for $100. I added a factory Parkerized 870P barrel with a fixed IC choke and tritium bead for $100.
I adjusted the rear sight when I patterned the gun on the pattern board, used that to get close for FliteControl slugs, and then zeroed it with the slugs. It is now the 870 I use the most, and I have even used it to shoot skeet as practice running th gun. It does balance a bit too forward due to the weight of the light, but it is really dead in the hands with a loaded side-saddle. As such, I do not keep any spare ammo on the gun as I want the gun to swing smoothly without effort.
Pretty good shotgun for less than $500 invested.