I just made this same decision not too long ago. I chose the Sk-7. Why? Because the difference with the same fan would be no more than 2-3C really. I got the SK7 for 15 from SVC. They charge 30 for the SLK-800. Worth it? No. If you're gonna go for the slk-800, you might as well get the 900 and go all out.
Edit: Also, I love my Sk-7. I just upgraded from my AX-7 ( a sort of small upgrade you could say) but I already love it. It seems smaller and more sleek than my AX-7 (I love the thin fins) and it was't hard at all to install. The only thing I don't like, but like at the same time, is the fan clip system. They kept coming off as I was putting the fan on, but once in place, they are great. And not using screws is nice (the AX-7 has a crappy well of installing fans). I am already seeing the drop in temperature too.