I have recently aquired the UCA. I used the DCA for a long time but cameras compatible with that are getting scarcer and scarcer, so I decided to try the UCA. The DCB looks like too much kit to me so I have not tried it.
UCA Good Points: Once attached to your eyepiece you have the flexibility to put your camera anywhere so you get the most out of it. The adapater is somewhat bulky, but once attached to the scope is very solid.
UCA Bad Points: 1) One reason I loved the DCA was you never had to worry about the camera twisting out of perpendicular to the scope. The UCA, like most universal adapters I have tried, has this problem. There is nothing to keep the camera from twisiting on its tripod adapter screw. You have to check this everytime you mount the UCA. It needs a bar or something to keep the camera perpendicular. 2) The other reason I loved the DCA was that it was so easy to slip on and off. On my 25-50X eyepiece the fully loosened adapter is still a fairly tight fit and takes some jiggling to get on especially past the rubber zoom control. I wish it was a few millimeters wider so it would slip on more easily. 3) Finally the DCA was not very bulky, this is, so it is harder to carry around.
If you get a compatible camera the DCA is the way to go, but they are harder and harder to find. The UCA is OK and does allow you to position the camera perfectly (not always possible with the DCA). If you are primarily a digiscoper, the UCA is probably a good choice. If you are primarily birding and want something to slip on quickly for a photo, the UCA is OK but not as good as the DCA in this regard.