I don't shoot either one, but here's some thoughts on the two models mentioned.
Get a 12ga. It's a very useful and adaptable sized shell for a varieyt of hunting situations. A 20'll do it close, and a 10ga will do it with authority, but I kinda like to have sumthin left to eat.
1) There's a heckofa lot of 870's out there. Easy to find a used one in good condition.
2) It seems like Mossbergs are on sale a lot. It may be easy to get a brand new one for a good price. A lot of guys like the Mossberg 835 for their turkey thumpers.