870's are much better shotguns than the Mossbergs. The Mossbergs are alright if you want a nononsense shotgun that might be used for occasional use, but if you plan on using it alot, get the 870.

AS for 3 1/2 magnums, I'm not convinced that they are worth it. I've killed many ducks and geese over the years with a 2 3/4" 870 wingmaster and finally got a 3" shotgun a few years ago. The 3 1/2" shells would probably work well if you have the discipline to use them, but by in large people use them as an excuse to sky bust at longer distances. Because of the longer shot column, 3 1/2" magnums string more than 10 Guages and I don't believe they are nearly as effective as 10 Guages for longer range shots. The 3 1/2 would make really good turkey guns though as stringing isn't a problem when shooting at a stationary target.