I like it better than GTA because the online mode is a lot of fun; lots of different types of games.
Single-player - In GTA you're a "bad guy," in RDR you're a "bad guy" with a moral conscious. It's a lot more robust honor/dishonor system like Fable but it tends to reward/punish you more. The world is totally open to you from the beginning so you can just roam the land. The story is great and there's an easy travel system so you can fast-travel to any location you want.
Multi-player - You start every game in "Free Roam" which is the same world as the single-player but you can freely interact with other online members, kill them/respawn/kill them/die/respawn/etc, you can take on AI gangs and get weapons upgrades that last your time online, gain experience to get better weapons, horses, gear, costumes, etc, posse up and take out the computer or other gamers.
Then there's the match games; capture the flag, PvP, free for all, etc.
I just had a lot more fun in the Old West than I did in another GTA clone city.