Saints Row 2Saints Row is a barely concealed rip off of GTA. Set in a large world, its based around following missions where you commit criminal activities, and if you get bored you can rampage round the open city. The difference is definitely in approaches. GTA, especially 4, has become more focused on story telling, and, for the most part, succeeded very well in telling a story about a character we cared about. There was some mild moral dissonance where you got the feeling that not all the writers always talked to you, but it was a fairly gripping story. Along with this came a system that was a lot more realistic, with defined characters, more real physics and a combat system that punished the gung ho attitude of former games.
Saints Row 2, however, is not going for realism at all. Its going for insane, over the top grandeur. Its going for a ridiculous plot, and a main character who is able to do ridiculous thing. You are able to sustain massive amounts of injuries in Saints Row, and you even regenerate your health. The cars fly around like toys, and the weapons selection is amazing. Its awesome fun, and feels like a massive improvement on GTA:San Andreas, taking many of the ideas from that and making them much better. For example, in San Andreas you could capture territory, but the mechanism was terrible- effectively you started shooting people then had to kill a gigantic army by yourself (who would usually run at you up the street from either side). It was more than a little stupid. Saints Row has the same idea, but makes the territory capturing into missions, rather than randomly generated bad guys. You still have to kill lots of enemies, but usually in environments that make sense, with clear objectives that make it feel a lot more purposeful.
You can also buy property, which is mercifully cheap, unlike San Andreas where you could only think about doing it much later in the game. The only flaw here is that the idea from Andreas, where money has to be collected, is re-iterated, and adds a tiresome part of repition to the game that doesn't feel necessary. At least all the money is in one place, rather than outside the property you purchased!
Is Saints Row 2 better than GTA4? I don't know, they're both terrific games, but each celebrate something different. I'm not convinced that SR2's open world is more fun than GTA4's, just different. The missions are certainly more interesting in SR2, and the number of distractions are astonishing, but the reality of GTA4 add an extra verve to the game. I suspect that if you like one, you'll like the other.