You seem to have chosen the Markins but it depends on what gear you're using. If you're using something long and/or heavy go with the Markins as it has a 38mm diameter ball versus the PC 33mm ball. I use a 17-55/2.8 zoom which quite honestly is a bit of a challenge for the Markins and it has an excellent friction control system. The PC fares even a bit worse with it but a bigger ballhead has no problems at all. Clearly the size of the ball is a major choice factor.
OTOH if you have light to moderate gear the PC is a very nice ballhead indeed, it is a heck of a lot cheaper and it has a feature the Markins doesn't have; a friction scale ring on the ball locking knob copied from Arca-Swiss, see the pic below.
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This scale ring is used as a friction reference only and rotates with the locking knob as you turn it. It's very useful if you have a number of different rigs with varying weights. You set the friction tension with your lightest rig, then rotate the scale ring so that zero is at the index mark for that tension. When you put a heavier rig on, you just increase the friction until it's correct and then take a note of the scale number. Next time that rig goes on the head you just turn the knob to that scale number and your friction is all set; no adjustment necessary like with the Markins.
The Photo Clam has a locking action that is slightly less smooth than the Markins, its fit and finish is is excellent and indistinguishable from the Markins, and the drop notch is in a different place on the two ballheads. Take a look at the pic above, to me the drop notch is at the front on the Markins, at the side on the PC.
They are both very nice ballheads and it's a tough decision. User reports on the PC have been good on many forums. I think the Photo Clam is better value than the Markins and I'd definitely choose it if I had lighter equipment.