The first one is simply an amazing game. Theresa and I take turns playing levels and we had been playing for the last month nightly to get ready for the new one to come out.
SMG2 is pretty much an extention of the first, but with lots of little tweaks. Besides the new suits and levels, the gameplay is a bit more fun. Your home is now a Mario head spaceship made to look like a planet. That is how you travel to new worlds. It is really easy to get to any level and you do not have to do all of the running around like you did on the observatory on the first SMG. Yoshi is a fun new twist. His tongue and movements add even more complexity to getting around.
We spent so many hours on the first game and it is a great way for Theresa and I to hang out together. SMG2 seems a bit like an expansion pack, but it still has 241 levels like the first and is a new adventure. Grab it if you liked the first.