I wanted to like Unit 13, but you can't unlock stages through co-op play. That's a massive oversight for a co-op centric game. I'm just glad that isn't the case with LBPV, was very fun to play through the campaign with my brother.
I still have MGS HD Collection sitting on my shelf. Afraid if I start it won't be able to put it down. I remember beating MGS2 6 times back in the day.