I haven't been to Playa Del Carmen, but we've done both the resort courses and local guides.
My experience with local guides is that they'll take you to some of the best places for the little things - tiny commensal shrimp, nudibranchs, etc. They also seem to have more passion for scuba diving in general. Their equipment is usually better and better maintained since its their business on the line.
Resort course guides seem to just want to show off the big stuff that makes everyone go "ooh and ahhh", like sharks, dolphins, turtles, etc. Equipment is usually adequate, but not great.
Overall, I prefer the local guides, but if you get to know the resort guides (especially if it's all-inclusive...might as well take advantage of what you're already paying for) then you can build up a relationship with them. We paired up with a resort guide who often took us beyond the "usual" tours.