I have used both and have gone back to the MEC bushings once I got my load dialed in. I saw some powder drop variation with the universal adjustable bar. I haven't done an exhaustive study, the weight of powder dispensed just seemed to vary more than I liked. The only explanation I can come up with is the shape of the powder cavity. It is useful for load development, though.

Phil, I probably shouldn't say it, but in 12ga 15grs of anything is pretty safe. If one looks through all the different powder manufactures reloading handbooks, 15grs is a light load. The old manuals use to list everything for fiber wads and when plastic wads first came out they said to go down 10% if using plastic. That kind of tells you subbing in fiber for plastic will have lower pressures.

Instead of a Universal Charge Bar I have a "Shuttle Bar" made by the same company. It has an adjustable shot cavity but uses standard MEC powder bushings. I picked it up in the "used" bin at Kittery Trading Post many years ago for $12, I had not heard of them until I found it. It has been very handy for reloading Bismuth shot for my waterfowl loads. They still make them. It does seem to leak a little powder as many here mentioned about the UCB.

http://www.multiscalecharge.com/products_shuttlebar.php