If you don't need multiple installations, go with one. If you don't know if you need multiple installations, go with one until you find a compelling reason to change.

I usually run 1:1 copies of WoW to machines, but I only have four screens immediately in front of me, so when I five box alone I do run two on one machine.

For that machine, I chose to have separate installations for the flexibility of keybindings and to avoid any possible contention when writing the shared cache files, having heard anecdotal evidence of problems there.

Load time is trivial, and not noticeably slower than my other machines to be honest. Entering an instance sees them all pop in at pretty much the same time.

But see my first paragraph