Its the same game as MGS story wise except for some dumb action movie stuff, and you'll be rehearing a ton of the same dialogue so if you play it immediately after MGS1 you'll be a little burnt out.
People love to exaggerate about how bad it is, but MGS2 gameplay is very fun so Twin Snakes is still pretty enjoyable. I would save it for after 2 because you'll be coming right off that game's gameplay and feel right at home, and it'll be long enough since MGS1 for the repeated story to feel a bit fresher
Looks like this gamer's paradise has turned into a gamer's hell...............................................