You wont be disappointed with the Mossberg. I have a 935 that I love. Yes, it gave me some headaches the first few times I had it out because it would cycle slowly and stovepipe. But once it was broken it, all those issues dissapeared. It probably took about 50 rounds to work in the gun. I worked in the gun with Winchester 2 3/4" lead game loads. Now, my 935 will easily cycle 2 3/4" shells (including light target loads) even if Mossberg says the 935 wont. Its a great gun. If I had to do it all over again, I would definately buy another one. I chose the 935 over the 930 because I wanted to use it exclusively for goose and turkey hunting, which sometimes the 3 1/2" chambers can be of benefit. I only wish Mossberg made the 935 with wood stocks as I've never been a big fan of synthetic guns. A couple of hints if you decide on a 935...use Breakfree CLP oil to clean and lube it. For some reason my 935 prefers that brand of oil, especially when its really cold out. Also, my 935 doesnt like the 3 1/2" Remington Hypersonic loads. Those loads are a bit longer once fired and that extra length of the spent haul will cause hangups. So if you prefer the Hypersonics, stick to the 3" loads. All other Winchester, Federal, Kent and Challenger loads cylce fine regardless of the shell length.
There are also a good bit of accessories that you can buy for the Mossberg's. Since I purchased the gun, I've also acquired a cantilever rifled barrel for deer hunting and a choate pistol grip stock, which was an awesome addition for turkey hunting. Its now my go too gun for pretty much every type of hunting that I do.
As for the gun slowing down when dirty, I can't say that I've ever experienced that. Im sure all gas guns will slow down at some point if not cleaned, but I've always cleaned my guns after the day ended so I've never had this issue. The Mossberg is very simple to clean. It literally takes me 5 minutes to fully clean it, 10 minutes tops if I take my time. The only tool that is required is something to punch out the two trigger pins. I use a punch with a wide handle, and knock out the pins with my hand. Everything else required to field strip and clean comes apart by hand.