I'll answer your last question first. I feel like if you're carrying for self defense you should train with it either way (admittedly, I haven't had any official training); however, I think simpler is better.
I had to look up what a karambit is and can't speak for them. I have a very old butterfly knife that is of medium quality, but is very worn. What I mean by medium quality; it is no where near Benchmade balisong but better than standard 5 dollar carnival junk. It's fun to play with, and I can work it quite well. It would make a nice hole in someone if it came down to it; however, I don't feel it is in the same category as my Spyderco Endura. It has a long skinny blade that would be good for stabbing, but it is not nearly as substantial of a blade as my endura. On top of that, I can open my endura and have it in a usable position much more quickly. I think your asking in a mostly defensive context, but I want to add that I would much rather have my endura (or most any quality "pocket" knife) in an emergency "I really need a knife" situation. To me, a good simple knife is much more dependable and sturdy than a butterfly knife.
As a side note, you're a lot less likely to have legal issues with a standard pocket knife than a butterfly knife.