The main difference is in how they look when it's done - they kind of lean in opposite directions - K2TOG leans to the right, SSK leans to the left. The two slip stitches change the twist to the yarn (i.e. whether the left or right end of the stitch is towards the front of the needle before you knit it), and then the knit stitch is through the back of those two reversed stitches, so the effect is that the other stitch is the more prominent one on the right side when you're done. (At least, I think that's what it is. I just did a test swatch to see what happens and that's my theory.)

To me usually K2TOG looks like one column of stitches got eaten by the other one, while SSK looks like two columns smoothly merged into one, but I think that's mostly because I'm usually better at SSK because slipping makes the stitches a bit looser so it's easier to get in there and knit them together. So the two connected stitches lie a little more flat, and any bunching that happens is mostly on the wrong/purl side of the knitting.

But it doesn't really matter as long as you have a decrease.