Pros: It's gorgeous. The ruins are set high on a cliff overlooking the Caribbean, and the gardens are well maintained. If you're staying in Tulum, it's a short drive or bike ride away and if you get hot, you can swim mid-tour—just take the stairs down the hillside and step right into the crystal clear water. El Castillo, the city's temple, is the main attraction but there are many other small painstakingly restored structures to check out. Overall, the site is compact and can be seen in under an hour. As the only settlement the Mayans built near the sea, it's definitely worth a visit.
Cons: If you arrive in the mid-morning or early afternoon, expect crowds. Cruise-ship crowds. For the hippie-leaning souls who dig Tulum's resorts, this can be quite a culture shock. Going soon after the site opens at 7 am, or after 3 pm will give you more breathing space and also lessen your chances of sunburn and dehydration (oh yeah: bring water).
Pros: The rainforest! What you lose in ocean views, you make up for in overgrown, lush vegetation. A 30-minute drive from Tulum means you get to escape the cruise-ship crowds, and the larger sprawl of this site gives you less of the touristy feeling. The ruins here look less pristine than those at Tulum, which for us only added to their appeal. Most of the site remains unexcavated. For now, you can still climb Nohoch Mul, Cobá's pyramid, and, best of all, you can ride a bike (or be peddled around) from ruin to ruin. You're also more likely to spot wildlife here than along the heavily-trafficked paths at Tulum.
Cons: The only downside is the distance, but if you build in enough time, that's easily taken care of. In the summer, the jungle setting can make for killer humidity and in the rainy seasonwell, you get rained on. We encountered a downpour during our visit, but riding along rocky, muddy dirt paths past structures that are 1,500 years old made us feel way more adventurous than we really are.
Both sites are easy to find if you're driving, but it's just as convenient to ask your hotel to arrange a day trip for you. Our hotel Rosewood Mayakoba uses a great local company, 4Worlds Expeditions, whose tours include your transportation in an air-conditioned SUV (complete with mariachi soundtrack), a guide, entrance fees, and (ah!) cold towels, water and soft drinks in the car. All sorts of add-ons are available, with tours to Tulum starting around $110 per person.