Congratulations and welcome to the forums! Personally... I prefer the Playa del Carmen area just because it is a great little town that have something for everybody! Guests can always head into town for some shopping, entertainment, beach of just drinks. You really don't have to worry about them being entertained. As mentioned above... Tulum is much more tranquil and very relaxing. It all depends on what your looking for but I think being closer to Playa will make for a better week for everybody.
Best of luck with all your plans and feel free to email me directly if you have any questions.