The Martin J-8 has an advantage over the Bitz, in that it has a micro-adjustment that allows you to precisely position the feathers at the angle you want to the nock. This is an advantage for those of us who like to use the cock feather against the nose anchor, and are particular about having the cock feather at a certain angle.
The Bitz has various indexers available, and some indexers have blank rows where you can drill your own holes, so you can eventually get the angle you want, but it is not as convenient or as fast as with the Martin.
You have to be a little more careful using the Martin to get the shaft positioned correctly in the slot, whereas with the Bitz the shaft pretty much falls into the correct position by itself.
My Bitz's have long been set up with indexers that give me the angle that I want, and as long as I can get that angle without any hassle, I prefer using the Bitz because it is more sturdy and the magnet is stronger, so seems to give me more consistent results, or at least requires less of my attention to get those results, than the Martin.
I'm fine using either one, actually, and keep my Martins set up for one diameter of shaft that I use and my Bitzs set up for another diameter.