I would say that there is really very little difference between the two games, but I did prefer one over two. It seemed more "survival horror" like and less "action shooter" ... even though I would say Dead Space 1 barely counts as horror.
If you want to get an idea of why Dead Space 2 is inferior then you need look no further than the addition of competitive multiplayer. The multiplayer itself is not horrible, but it is the idea behind it that reveals a lot about the development process of the game. Much like the Mass Effect and Dragon Age series, EA is desperately trying to move away from the niche genres (horror, and RPG) towards the much more popular and lucrative action/shooter genres. The great thing about action shooters is that they can be released year after year with little to no changes all while charging a premium for map packs and DLC (the Call of Duty model basically).
I expect it will get even worse with Dead Space 3: Modern Warfare in Space 1. Now multiplayer is coming into the campaign. I can't remember what the name of your partner is, and I don't think even the development team knows his name or function in the story, but he will be there with you every step of the way during every shootout ... just to make sure you never have to get scared or anything. I think I will call him Soap just for simplicity's sake.