One facet of the M500 family is the safety and takedown. Vertical grip stocks, in every case require the stock bolt be used to secure the grip, thus a tool is needed for disassembly, one thing to consider.........
another is the position of the safety, on the tang. It can only be readily used from a standard montecarlo style stock......so if you use the safety, such as for hunting, or breaching a door(haha).......a pistol grip can be a problem.
The flex system seem to have addressed this takedown issue, but not the safety issue.
You can also get the conversion kit as opposed to getting a new gun.
The above is one thing that makes the 870 a more versatile gun overall in regards to functionality with different stock options. I still find the M500 family the best pumps on earth, but everything has its advantages and disadvantages......