Hard to say as Joyce is still relatively untested. Miller, weak though his resume is, is a few steps ahead of him in this regard. A few things we can be sure of is that Joyce definitely has the higher (or at least more consistent) workrate while Miller has the marginally faster hands (albeit in bursts). They're both incredibly cumbersome and sloppy though, with some of the worst defence I've ever seen. The fight would be one of those non stop slugfests that might be entertaining but might also resemble two drunks swinging away in a car park.
Would be a cracking fight, neither wanting to take a back step, both throwing a ton of leather. It will turn into a battle of attrition and I think Joyce being fitter and hitting a bit harder will come out on top.
This. Really hard to say as we've not seen Joyce in with decent opposition yet. Miller is cumbersome and technically average but overall he is decent by current HW standards. I've love to see this fight.