I think from what I read in an interview, Guerilla was more open world, which people liked. The problem with that, is that the destructable environments were toned down because of that. I guess they were still totally destructible, but more like blow the hell out of a shanty town instead of take out the side of a mountain. Armageddon tried to fix that a bit by making the game more linear...which I'm assuming people didn't like so much.
That was what I got out of it anyways. Could be wrong, but I played the demo of Armageddon and it seemed pretty fun. Have yet to install Guerilla though....I got it in some THQ hits collection in a Steam sale.