Earlier this year, I went searching for a new HD SG. It was difficult but my final 2 platforms were the Remington and Mossberg platforms. I put several models of each in my hand (didn't fire them), looked at the pros/cons of each for my situation, and determined the Mossberg platform was best for me. From there, it got even tougher because there are soo many good Mossy models to choose from but I finally ended up with a 590A1. Great gun and I'm very happen with the choice.
Really, you can't go wrong with either but since you've got it nailed down to those to platforms, the choice really becomes yours. Good luck on your decision.
Smith & Wesson M&P 9 FS (w/ mounted TLR-1)
Smith & Wesson M&P 9c (w/ Crossbreed Supertuck IWB)
Smith & Wesson .357 640-1 (w/ Galco KingTuk IWB)
Kahr CW9 (w/ Kholster Crescent IWB)