Well, I own just a few SKs. For the money, you cannot go wrong with a Barry King SK. Some of the top makers are Paul at Leather Wrangers, Bob at Pro Series Tool (Black Crack), Chuck Smith's Ol' Smoothie, Clay Miller at CM Tools & Leather. I hear that Bearman (a member here) is making SKs now though I do not own one yet.
Now there have been a lot of threads on here about SKs. You can search the forum using the section at the top right corner.
In my opinion, it all depends on your level and what leathercraft means to you. Is it a hobby or a business? If you are just moving up slowly, you could be fine with just upgrading your blade from one of the makers above. I think Barry King offers the most affordable blade upgrade at about $15 and he makes them for Craftool SKs. There are some other companies that offer middle of the road SKs. Seiwa, CraftJapan, Kyoshee Elle, and several other sellers that can be located through your local leather craft supply store. This should help you make the appropriate decision.
Good luck!