First of all, we are talking about two distinctly different styles of knife here. Both defensive knives, but very different.
The Desperado is cool, I wish the blade was just a little shorter, not serrated and at least partially double edged. I wish it was still in decent steel (not that 420 wont hurt someone, but.. yeah..) and with a real kydex/concealex sheath not this molded crap. Sheath needs Blade-Tech type IWB loop, and Tec-Lok attachments for versatile carry positions, horizontal cross draw, strong side draw, etc. etc.
Its a thrusting weapon, you'll do better thrusting with it than cutting. It does allow versatile grips, and with comfort. Also, it gets around push-dagger laws because its not a t-handle, its just a "palm-knife", that happens to be able to be held like a push dagger.
A karambit, B.S. and hype aside has two things really going for it, Retention and a hook blade.
I am a
Pikal(Edge In reverse grip) kind of guy, say what you want about that, I like it, its nasty, it works for me, quite well. BUT, if I have to use an edge-out blade, I want it to be a hawkbill. It hits point first, stabbing of sorts, going deep in and THEN cutting, make a deep wide wound, unlike a knife with "belly", which can hit and just sort of skate along never bite in as deep. So, if you gotta do edge-out, I figure a hawkbill is as good as it gets.
Now, you've got the finger-ring. Hands covered in sweat, blood, fluid from the bowels and stomach, etc. etc. That ring, as long as your finger is in that ring, your hand is going no-where. You have retention up to wahzoo, but you can also let go of it real fast if you have to (not sure why you would want to though).
Screw extended position, thats fcked up. Also, if you carry both fixed (double edged) and folding (single edged by nature) karambits, it breeds bad habits and int he heat of a fight you might smack someone with the spine, and have your folder close up on you.
Now, I do like "hammer fist" type chopping with the back edge of a double edged kerambit, its a natural motion, and its like taking an ulu to someone I'm sure. But, I wouldnt want to do it witht he un-sharpened spine of a folder (or even the spine of a double edged folder).
So, realistically, sensibly, what is a kerambit? Its a hawkbill fighter, with above-par retention.
Now it comes down to, how do you fight, and which features from which knife are more important to you, which one fits your carry profile better, which one can you use better, and so on and so forth.