Hi - It’s been a while since I last tried something like this.
I’m roughing up a concept for a friend who has changed their mind regarding the figures arm. For speed I want to either mirror the geom so I have another arm & move it to require position or insert a new arm. Which is the best method?
there is no “better” method.
The only differnece is which one is faster for you to work in. Mirroring an arm and welding it, duplicating the arm and moving it over. Dynamesh them together…all of these are usable functions, it just depends on which is faster for you.
personally I would probably mirror and weld, then qRemesh, but I don’t know if your mesh is unwrapped, aleady rigged in another application, etc.
Thanks so much for your input. I went for the mirror and weld option in the end. 
I don’t get a lot of time to go through the latest developments in Z-brush… I’m all too aware that some of my work flows may be becoming a little antiquated.