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ectoplasmosis-
The main structural reasons for PM over IS is that the materials of bolt and mount are better suited to absorbing the braking loads into the fork or frame than with IS. The bolts themselves are loaded in tension which is where they're strongest but on IS mounts they have to deal with a shearing load which is where they're actually the weakest. PMs also require less material to be used in the frame/fork construction and so you end up with marginally less weight. I believe when Fox went from IS to PM in 2008 they claimed something like an ounce of weight reduction in the lowers.
https://www.pinkbike.com/forum/listcomments/?threadid=127672
Tijmen
The most efficient way would be using an IS mount with a PM adapter. But it's not necessarily the most fun way.