I have the Bethwaite harness and Aluminum spreader, and have used the Kevlar spreader. Get the Kevlar, as it is much lighter. The AL one is comperable weight to a convention steel wire spreader.

The spreader is fantastic for crawling back on the boat post-capsize. I've had 2 rough capsizes in the last few months, one with a hook, and one with a Bethwaite system. No worries with the keyball, but my first thought with the hook floating under the boat in zero visibility and 7' seas was weather I had lines around the hook. (Next thought was "which way is air?")

I find the keyball system to be as secure as a hook ring, but you can't beat the ring for hooking up blind. (Sometimes I miss with the keyball and have to look down to connect, which should matter only to serious racers.) It's worth the safety, IMHO.

I find the Bethwaite harness to be as comfortable as any, despite the simpler design. And there are no straps to catch on anything. The spreader also works fine with conventional harnesses, in my experience. However, the zipper of my previous front-zip drysuit sometimes blocked the ball from hooking in easily.

The velcro on the Bethwaite harness is not reliable after it ages, even when rigged according to the instructions. Once you get it adjusted, have your sailmaker (or local apholstry shop) flag stitch the straps.

I've heard bad things about both the plastic (non-bethwaite) spreaders and wire keyball spreaders failing and dropping people. I would avoid those.