If you're not insistent on using a full, standard rack, I'd recommend the
BMS Micro-Rack. This is what I use, and it has always worked very well for me, with different pit lengths and different rope diameters. The bars are steel, they wear slowly, and the ride is smooth. I weigh 143lbs. The longest single drop I've done with it is Surprise Pit in Fern Cave (437 feet). I've heard that once you add about 100' more to that, it can be really hard to move. For 100' drops, the micro-rack will work just fine, and it's considerably lighter than a traditional rack, too.
I use the long-frame, and that is what I'd recommend to anyone who doesn't weigh a lot. You can always push the bars closer together, or even use the hyperbar. Dual-hyperbar models are available, but I have never seen the point of that. I use the one with the single hyperbar. I have found it elegant, easy to rappel with, and ever since I learned the Bassett method of locking it off, quick and easy to lock off. It has worked well for me for controlling two-person loads, too, and as a fixed brake for lowering.