The Mossberg 500 is a fine shotgun of course. The pistol grip is not very useful. I bought a Mossberg Mariner that came with both the full buttstock and the pistol grip. I tried the pistol grip thinking it might be handy for in the tent when camping. I took it to the gravel pit to pattern/sight in and it was very hard to hit anything past 10 ft. At 20 yds I could only get one or two pellets nicking the 2'x3' board. And it is not easy to manage the severe muzzle rise. If you practiced with it a LOT I imagine one could get proficient with it. It was not worth it to me to put in that much time, effort and expense into something of marginal value.
I put the full buttstock back on. If I need something for that close range I will use my G27 or M29 handguns. If I need a shotgun I will use the buttstock.
Bottom line, I would get the Mossberg as it is a fine shotgun for field (with longer and choked barrel) or home defense with the idea to replace the pistol grip. I would not get it just for the pistol grip.
"The ultimate authority ... resides in the people alone. ... The advantage of being armed, which the Americans possess over the people of almost every other nation ... forms a barrier against the enterprises of ambition."
- James Madison