I love my P30SK, but it's not exactly a slim or svelte handgun. It's about as large as I would want to carry; I wouldn't want to carry my P30.
Now, as to how they are similar, they are quite so. There is one notable difference, though: my pinky just barely fits on the flush baseplate on the P30SK, and there's plenty of grip on the P30 (oh, and the stippling on the P30 is a little more subtle and comfortable to me than the SK, for some reason). Other than that, I really don't notice much of a difference when I am actually shooting them.
You may end up with both. I did.