I have to go with the Rem 870. Not that its any better than the Mossberg. But like you, its what I cut my teeth on and it just feels right. I have 2 20s and 1 12 in the 870.
My friend has a Win Model 12 and a Mossberg 500 and they just do not feel right to me. Again, not saying they are bad. Lord knows the model 12 has a great reputation in the field and on the trap line and the Mossberg is a working man's gun. But again, so is the 870.