A Mossy 500 is a Mossy 500 as far as mechanical items are concerned. The Crown Grade will just have nicer wood and perhaps some engraving.
500A denotes 12 ga. and the 500C is the 20 ga.
A 535 is a 535 mechanically as well. There are a number of versions with wood stocks, synthetic stocks, blued finish, camo finish etc.
My opinion is you can't go wrong with the 535 as a multi-purpose hunting platform. I bought the Turkey/Waterfowl combo from Dick's this past winter before turkey season. They had it on sale for around $319 if I recall correctly, complete with the two barrels and a TruGlo red dot sight.
I use it for turkey, waterfowl and deer. I took my first turkey on opening day at 42 yds. using Remington Nitro Turkey loads. Of course I spent some time on the range first to learn how the gun patterns with different types of ammo. For deer I use the turkey barrel with an IC choke tube. At 50 yds. it'll shoot cloverleafs with the inexpensive Winchester 2 3/4" slugs.
I'd suggest you check out http://www.mossberg.com
where you can view their entire line.