I’m currently using PD-M647 pedals. My criteria were the same as yours and I considered all of your options. I ended up going with the PD-M647 because it is listed on Shimano’s website as a mud shedding design and the others are not. (The fact that I found a barely used pair on ebay for a great price didn’t hurt either.)
I’ve got no complaints with them. I’ve never had issues clipping in or out. The resin cages are looking a bit beat up now, but it hasn’t affected functionality.
I have the DX pedals as well (PD-M647). I’ve had them for a few years and have no complaints, although they are also my first set of clipless pedals so I have nothing to compare them to.
I’ve used the 647’s for 3(?) years now, and they’ve been great for me - trouble free, unlike the Crank Mallets which I liked, but gave me some issues. I might have bought the 785’s if they’d existed when I bought the 647’s. I expect they’d be lighter, they look cool, and since most of my riding is technical XC with no big jumps, I probably don’t need a big platform.
I have the Deore version of the 785’s and I am quite happy with them (M530 I think). There were the first set of pedals I used with any sort of platform and it was a nice addition. This may be be an unfounded worry but I always wondered how much more you would clip pedals with the large platforms (like the 647 and 545). When clipped in the 530’s sit fairly close to the shoe. Also, would the durability of the plastic cage on the 647’s be an issue?
