I've had the best luck with the plain Jane diaphragm type. When you start out, they both seem okay but I've had a couple of flat diaphragm type that over time (10 or so years) just seemed to go all weak and start slipping. Especially if you haul stuff with your truck.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end