I have been using a different tether for a while so I don't remember specifically the diameters. But, for sure I remember the prussik being hard to move once you've weighted it. I think that's just an issue with prussik....changing diameter may help...but I guess arguably on your tether of all things you should prefer no slippage vs. ease of movement.
Ultimately, I switched to 8mm Oplux and I use a kong duck. I know many would feel its unsafe to use an ascender on your tether. I have a whole backup system I wont' go into (2nd tether and RC harness under the JX-3). So, I'm OK using an ascender. I really like to be able to adjust one-handed. Often I'll sit down in the tree and get the tether all weighted and feel like I need to move up a bit. I have already taken my LB off. So, it's nice to be able to use one hand to steady yourself on the tree and one hand to adjust...so that's why I use an ascender. If I didn't have a backup I'm not sure I'd use an ascender on my tether.....just something you need to decide for yourself. I tried using a prussik and a tender for a while to get easier movement, but I could never get that to really work.