Played through both with my wife who is very much not a gamer and struggles with controllers.
1 is excellent and highly recommended but its much more difficult to complete. It takes efficiency and control of the characters to get enough stars to even finish the game let alone any kind of "completitionism"
2 is much more forgiving. It has more mechanics and more interesting levels but ultimately is a much easier game if you arn't trying to get every star. I was able to pick up the slack much easier in 2 then in 1.
They are both top notch couch coop games, as good as any in recent memory but as to which one to purchase it depends how competent your players are. I would get 1 over 2 if you are playing with people who are comfortable with a controller and skip 1 if your playing with "n00bz" :P
EDIT: The DLC's are all excellent and just add more of the same with a particular twist, there is enough new content to justify the cost but you arn't missing anything special without them. The original games are plenty long enough on their own.