I disagree with saying that comparing an older game to a newer game is retarded/stupid/<insert random insult> as they are obviously in the same genre. I see it as a completely legitimate comparision, and bashing on OP because he wants a discussion on the topic is, in my opinion, an act of meaningless schoolyard bullying.
I haven't really played RDR, though I did play Gun and loved every single second of it. From what I've read of RDR, and seen while friends plays it, it should easily emerge on top in most comparisions. While in this matter as a whole new equals better, that does not automatically apply to every aspect of the game. It's quite possible OP wanted a debate of which aspects were better in GUN contra RDR.
Just my 2 cents..