I find the Mossy safety is in a more natural position to activate without changing hold, and tends to keep your thumb up on the tang instead of wrapped around, where it can bump your nose. Not an issue for experienced shooters, but many a noobie (student) becomes gun-shy after banging their nose with their thumb wrapped around. Also, students tend to have to look for the safety on the other two, but use it instinctively on the mossy, since the thumb rests on it. I have Mossy 500's in 20ga for my wife due to this.

If you've got the 870 action down to second nature, it probably makes seen to stick with what you know. Those were just my preferences and opinions. Practice with whatever you get, it'll do fine.