I really wasn't too pleased with GTAIV. The graphics were great, but because the engine was such a staple in the actual coding of the game they had to remove a lot of things that you could do in San Andreas (i.e. get tattoos, cut your hair, all the gang wars were gone etc), and that's what made me rediscover my love with the GTA series. I hear the expansions to IV make the game a lot better -- in fact I've heard the stories in the expansions are better than the actual game its self.
Red Dead on the other hand, bugs aside, has a great tale of redemption, family (who don't call you every 30 seconds to go to a strip club, and when you say no their approval rating drops), and status. The graphics aren't as great, but the content is much more which is why I don't mind that they toned down the engine