I would say you want the type that uses gravity to grab the 'hook" clamp onto a vertical rod. I recently hung a show where the clamps that attach to the frame wires were screwed onto the vertical rods with a pair screws. The big advantage is that I could make precise adjustments to the height, lining up frame heights was a snap. The big disadvantage was forgetting to tighten them beyond merely snug, which was a little hard to do adequately without a pliers. The gravity-clamped types can sometimes shift a bit as you hang the frame, but once the weight of the frame is asserting itself it's automatically secure, no brain cells required.
One thing's for sure, you'll never go back to nails! The first time I encountered one these systems in a gallery it was a pure joy. I had planned on a grizzly 1+ day installation, but it took less than an hour. The heights were perfect, and there was zero impediment to tweaking the frames around for best spacing and effect.