Welcome aboard Sir!
Like SHOOTER said, your choice of stock type should depend greatly on your intended purpose for the weapon. Adjustable stocks with pistol grips can have their advantages, but can be less than desirable for sport shooting. I use both types, but for different purposes. Take a look around the 500 and 590 subforums and you'll find plenty of conversation on the pros and cons of each stock type for multiple shooting applications.
We've got a great group of folks here on Mossberg Owners and the atmosphere is very laid back. The only thing we like better than discussing each other's guns is lookin at 'em, so be sure to post pics of your 500 and any other firearms you own in the various sub-forums. You'll find that we're into a lot more than just Mossberg around here. Let us know if you have any other questions.
Enjoy the site!
- L.E.S.