Those are the two folders that I carry the most. I haven't really decided which one I like better yet after years of PM/PM 2 carry and a month or so of 566 carry. I'll tell you that the PM 2 is a bit friendlier in the pocket because of it's lighter weight. It's also a bit easier to manipulate due to the G10 on both sides and its light weight. ZTs G10 is fine but the SS lock side gets a bit slick and cold sometimes. OTOH, the 566 lock bar is very easy to work. I've had no problem with the lockup or lock bar on mine.
I like the A/O on the 566. Anything with thumb studs that's smaller than a BM 950 is hard for me to work with old & stiff hands. The PM/PM 2 opening hole is a good alternative to studs because of its size and location on the blade. I have no manipulation problems with my Paras (that's probably why I have 6 of them).
As far as steel is concerned, the Elmax on my 566 is now as sharp as any folder I have, including my Paras. After some Sharpmaker work and some time on my strops, the edge is smooth, even, and slices as good as anything I've got. I don't have enough experience with Elmax to talk about field performance, but I really like how it responds to my sharpening/polishing capabilities. My Paras are all S30V with one exception (S35VN). I actually prefer the S35VN blade, but I like the feel of my digicam Para 2 so that's the one I carry. S30V isn't nearly as friendly to me when I'm sharpening it as S35VN, Elmax, CPM-M4, etc. In fact, S30V is about my least favorite folder steel. I carry my Paras in spite of the steel, not because of it.
The best price I've found on a ZT 566 is from Kershawguy. I got my backup from him. He's a good guy to deal with and a BF supporting dealer. I can't tell you which way to go with your decision, but this is my experience, if it helps.
Oh... as far as robustness... the ZT feels more solid because of its material and weight. I like that, I really do. BUT... I've put Spyderco Militaries and Paras through a lot of crap in the 8 years or so that I've been using them. They've never failed, broken, jammed, clogged up, or run out of edge even once. No broken tips, no lost clips or loose screws, no blade chips, no lock failures. This includes warehouse work with cardboard, zip ties, and computer cabling, hiking and camping chores, helping a buddy remodel his house by adding a new room (wood, drywall, carpet, etc) and general use around my house & garden.