The engine depends on what you want. Both have pluses and minuses. An initial factor is how much money you want to spend. You can build a 350whp KA on a stock/refreshed block for less than a good SR swap. At this point the sr is doing ~190whp and you're out 3.5 grand. Of course the SR has a bunch of crazy japs tuning its potential and producing parts so ultimate top end power is easier to achieve on an SR... at a price.
What you should be doing is examining dyno graphs.
http://www.ka-t.org/forums/viewforum.php?f=3
http://www.sr20forum.com/turbo/82080-post-your-dyno-sheets-*turbo*.html
Since this is drifting forum, I'm assuming that you would like to have low end torque. The KA is a much easier engine to get 200 ft/lbs of torque at 3krpm than on the SR. I'm sure the SR could achieve it, but again, at a price.
Here's the easy SR vs KA buyer's guide:
1) What motorsport are you running in?
2) What kind of power do you want for that motorsport?
3) How much cash do you want to spend?
4) Go look for a dyno with parts list that meets your budget, and build from that.
In other words you need to figure it out for yourself. Everyone's going to try and sell you their belief as to what engine is "best", but what you need to determine is what engine is best.... for YOU. And there's only one dope good enough for that job...(hint: you.)
Also.. TUNE TUEN TUNE TUENE TUNE TUENENENENE. For fucks sake tune. I can't speak from experience, but from research, on the KA, the only need for forged internals below 400whp is detonation insurance, from faulty hardware(cheap wg), or, wait for it, shitty tuning!!
Did I mention tuning?