The PM / PM 2 series are cheaper than Emersons, that's true. Whether S30V is a better steel than 154CM is subjective and depends on the user's requirements and preferences. Personally, I prefer several steels, including 154CM, to S30V. Even on folders, I like a steel that's biased toward toughness and ease of edge maintenance over edge holding. Emerson's 154CM is easy for me to sharpen even after fairly rough use, it gets extremely sharp, and I only have to worry about sharpening one bevel. I've also found that I don't have to sharpen it as often as I thought I would based on comments I had read about 154CM before I tried that steel.
Since the 15 series is V-ground instead of chisel ground, I've not had any trouble with the few long straight cuts that I've had to make. Granted,the material is softer (usually long pieces of art material for my wife) but the cuts are straight.
I can't compare blade coatings because I don't often cut materials that rub against the coating. Most of my cutting is fairly narrow stuff or something very flexible that parts at the knife edge. The only DLC Spyderco that I've used is a DLC Para 1 that my wife keeps in the garage for a garden knife. It lays on the concrete and gets pulled through dirt, clay, etc. The coating on that blade is somewhat damaged because of the type of use she gives it, but I suspect that's not a fair comparison with normal use for most folks.