Markins ball head and plates
I am using Markins Ballhead (Q20), with Markins plate (lever). While I find lever faster than screw to lock/unlock, at the same time I regret not having screw version for a couple of reasons:
1. screw gives more control, as mentioned earlier screw can be slackened just a bit to give enough room to slide the camera w/o risk of it falling off the plate. With lever it is more hold/loose situation and with heavy lens (N 24-70/2.8) my camera almost fallen of the head while I slackened the lever.
2. I am using Markins' L-plate and Markins' replacement foot for 70-200/2.8. Still had to adjust - either lens was comfortably tight and camera was to loose (could not get sharp pics with long exposures), after adjusting gap to match L-plate, it is very hard to lock-close lever with lens mounted.
If I was to buy today, with experience I have it would be screw version for sure.
Few turns each way of cost, and priceless comfort of security of the equipment as a reward.