it's amazing how many people like to comment about SR vs. KA, and none of them have experience with either one. I would take Hugh's info over most everyone else here. The KA24DE-T most certainly does NOT "suck" as one ill-informed user put it. 630wHP with just rods/pistons (bigass turbo/injectors and GOOD tuning... no headwork! just cams...) on a KA24DET, and I'm sure everyone by now should know that four people in the US have knocked down over 530wHP on STOCK bottom-end SR20DETs. It's harder to make an argument for a KA24DE-T on a car that didn't come with a KA (which would obviously make the to-do list a bit simpler ), but it's still a darn good engine. It can spool a turbo quicker than the SR, and it makes more torque, and parts are a bit easier to find.
Of course, the SR is about 50 lbs lighter, with a bit of that weight further back (with stock exhaust manifold and the like), and the SR is ~140+ lbs lighter than the RB20/25 engines. The SR is more stout to begin with, but all you really need to do on the KA is a set of pistons... unless you want more than 550wHP, in which case you might want to see a shrink before you start turning wrenches hehehe. And either option will take noticeable weight off the front of an old-school Z-car, which will help overall power-to-weight and will shift some of the distribution towards the rear. Of course, then you're stuck with the not-so-nice 4-banger exhaust noise... my SR doesn't sound that bad for a 4-banger, but it's certainly no inline-6.