back to OP...
Two things to consider:
A) how quickly and reliably can I provide gas to a diver who is really OOG and needs it immediately?
B) how easy and convenient is it to exit, go through restrictions, swim in current, hold on an ascent line, do a free ascent, ...
Two very different aspects, the various applied methods have their pros and cons.
I think the long/short hose method often taught for sidemount is not good for reason A. I can't just primary donate like in backmount; I have to figure out first which reg I'm breathing, then make a decision as in 50% of cases I must donate the one clipped away. But in this situation A you want shortest reaction time by a procedure that's simple, fast and reliable, like permanent primary donate like in backmount DIR.
UTD Z-system or ISE/Toddy method both allow permanent primary donate. I don't like the manifold, hence it's ISE for me. Minor drawback (irrelevant IMO): when switching to the left reg, you unwrap and stow the right tank hose and clip the right tank reg on the left shoulder. As this takes me about 3secs and I do it no more than twice per dive, this is a very minor inconvenience.
On topic B, there's also the discussion of hose length, swapping tanks, ... it's quite independent from A. Once the OOG diver got a functioning reg in his face and calmed down, the buddy pair can sort it out for exiting/ascending. They can switch regs again to get a more convenient routing, swap stages or sidemount tanks, whatever they feel is best for the exit in their particular situation.