I have several of both. The Sebs are wonderful knives, but are available in only one style and one blade alloy. With Kit, on the other hand, you have a much broader array of choices regarding both design and materials.
One Carson folder (# 18) I have has Ti scales and a Talonite (r) blade. This knife is impervious to the elements; you could toss it in the ocean for 20 years, fish it out, wash off the crud, and it would be perfectly functional. BG-42, while a good stainless alloy, would not survive this treatment. Sebs, BTW, will never be offered in Talonite; I spoke with CR about this, and he requires that the blade material be ferromagnetic so that their magnetic jigs will work.
At the other end of the spectrum, I have another Carson folder which is made of Stellite 6K (r); it is constructed of carbon fiber (scales and spacers) with an inlaid Ti lock. Closed, it is slimmer and somewhat shorter than the # 18 Ti/Talonite folder, even though the blade is 1/4" longer. There are two amazing things about this knife: First, even though from the pivot screw to the tip of the blade there is nothing but Stellite, which is denser than steel, and behind the pivot point there is only a short inlaid Ti lock, four tiny Ti screws, and the c-fiber scales and spacer, the knife balances right at the pivot screw!
The other thing is that the total knife weight, with a 4 1/4" Stellite blade, is only 3 oz!
The ergonomics are better on the Kit Carson knives. Sebs are square and blocky by comparison. It is much more comfortable carrying my #18 Kit Carson folder in the front pocket of my jeans than it would be a large Seb; this despite a 1/2" longer blade on the Carson folder. In fact, I do not carry Sebs in my pocket because of the poor ergonomics. I use a belt pouch instead. I must respectfully disagree with Art Sigmon about the relative ease of opening; I find that the Carsons are easier to open than the Sebs. I think most people will find this to be the case; there is even a note in the Seb FAQ's about people who find Sebs hard to open and how they should modify their thumb motion to make the knife easier opening. I should state that both Kit's knives and the Sebs I have owned have all been very easy to open and close; Kit's are just a little bit slicker.
CR stands behind his knives, but does charge for refurbishment and repair (as do most manufacturers). Kit will repair and refurbish for free. I have occasion to send knives back to Kit, once because I loaned my Carson folder to my hostess while at her house for dinner. She wanted to practice her 'flipping' technique, after I had pointed out that she was doing it incorrectly. I should have looked at her BM 612 and 625 (Leopard Cub and Leopard) before loaning her the knife. The BM's were both destroyed; she had to send them back to BM for rebuild; the stop pins were even loose, and could not be tightened! Well, after 30 min. of flipping, she had actually put a flat on the stop pin of the Carson! I sheepishly called Kit up, and, with his blessing, returned the knife. He fixed and refurbished it for free, and it is now a year and a half after I got it back; it is my every day carry, and the action is still tight as when I first got it.
I rather like the stone washed finish of the Sebs' blades better than Kit's usual bead blasting; both Kit and CR will polish the blade for a small additional charge, but Kit sometimes complains about his aching shoulder and elbow for months after sending me a polished blade knife.
Bottom line: Sebs are wonderful, but the knives I carry are Carsons. In other words, if you like Sebs, you will LOVE Kit Carson folders.
Hope this helps, Walt