Not that I've ridden a 4-stroke KTM, but those early Euro discs were crap. I'd prefer a TT600 with the twin leader any day (any day when its not raining, at least).
The '84 XRs had the disc. Its easily the best brake of the bikes mentioned.
Locally, the class is a real mixed bag - Den's KTM, a couple of KLX250s, a TT250H (soon), Paul's "Kawasaki XT", Dan's XR500, the occasional HL500 replica, etc.