I seem to be alone on this, but I didn't really think the second game blew away the first game. I thought they were both pretty awesome. Though I think I kind of ignored a lot of the new "stuff" in the second game.
I should say this too-the Metal Gear Acid games have a bit of a learning curve. I actually think they're pretty easy once you "get" how the system works, but since they're pretty different from anything else, they're a bit confusing for the first hour or so, though they start you off on easy areas.
They're basically a strategy game, that uses a deck of cards to do movements, only has two characters to deal with, and also a stealth game...and it all works together really really well. They're like Advance Wars in their addictiveness IMO. And it's fun because unlike the "real" MG games, you get to use lots of your best weapons all the time, which is a lot of fun, versus always having to conserve all your stuff. They're also one of the only games of this genre I can think of that uses stealth.