If you are thinking about resale value ... don't. New guns lose value almost as bad as new cars. Get the gun you want and keep it, Very few guns appreciate in value unless you keep them a very long time.
That being said... S&W pistols hold their value better than the rest. Especially J-frame snubbies. You can get almost new prices for them. There seems to be no love for used S&W autos. They make terrific autos, but just not much a used market for them. Ditto on bolt action rifles, most are re-badged Japanese Howas and don't command a great price. I can't comment on S&W AR's. I really haven't kept up with the used prices on them.