Meh its sorta you're choice. Both are very similar, and can make the same amount of power. Sr20's can make 350+ on stock internals and be reliable while for a KA that's pushing it. Then again it all depends on your tune. Oil pan on a KA is a pain since it's one piece while the SR is 2 piece.
Another bit of info, the SR is much lighter than the KA which was my deciding factor.
As for the price, well you need to look for deals. for me it was easier to buy a car with the SR running and then part the car out. At this point I have paid $300 for my SR, and it still has an aftermarket intake and exhaust that I'm reusing. I also have a KA in my 240sx that I will probably try and do a minimal turbo setup on. but that won't happen for a while. My best advice for you is do more reading, and factor in how available both motors are to you.
Since I had to start from scratch, it was fairly easy to pick one, but if I just had a KA laying around then I'd use that. I also do a lot of reading on Zilvia, nicoclub, and 240sxforums