I love my 590-A1. 100% reliability and you can mod it to just about any thing you want. It is the "under the bed" item.
I had a FNH SLP with the 22" barrel and it is an awesome shotgun as well. Very smooth and quick.
I prefer a Mossberg over an 870 for 2 reasons:
1- Ejector is an easier repair on the Mossy.
2- Position of the action release.

I had played around a bit with a SPAS because I was thinking of buying one. Seemed really heavy and boxy feeling.
M-4 is a nice shotty but is it woth what you pay for? I dont think so when you compare it to the SLP or 930.