Right after I booked my week-long vacation to Cancun, I bought the Frommer's Cancun and the Yucatan 2012. A few weeks later, the Fodor's equivalent, Fodor's Cancun and the Riviera Maya 2012 showed up in my selection of Amazon Vine books. I grabbed that one, too, because many years of travel around the world has taught me that two guidebooks often help me triangulate on the best destinations. Since you're probably trying to decide which guidebook is best for your own trip, I decided to review both books together.
This can't be a deeply exhaustive analysis as I spent only a week in Mexico, with only one major excursion (to Chichen Itza, by tour bus). Plus, I kept mainly to the obvious tourist attractions (rather than independent explorations, as is far more usual for me). But I'll do what I can.
Unsurprisingly, there is a lot of overlap between the two guidebooks. The Hotel Zone (aka Tourist Zone) in Cancun is just-so-big, with a finite number of restaurants worth recommending and shops to tell you about and typical excursions. Unlike guidebooks for, say, Southern California -- where one book might tell you about a cool little art museum another fails to mention -- they generally point to the same places, and give their nods of approval to the same businesses. From poking at the listings for not-as-tourist-infested areas, such as Merida, it appears that there's more of a distinction, as well as a between-the-lines personal affection for the places to see.
The biggest difference between the two books is a matter of "author's voice." The Frommer's book is clearly written by an individual with personal opinions (one author per area), who imparts data as well as the reasons for a recommendation. For instance, the review of the JW Marriott, where we stayed, says, "This remains my favorite resort in Cancun, a refined oasis that offers exceptional service without pretense. Despite its many touches of elegance, the JW is friendly and even family-friendly -- although more families stay at the neighboring and less expensive Marriott Casa Magna." (There's lots more, including details about the rooms, the spa, and the restaurants, but that should give you a taste of the writing style.) Fodor's has a deal with TripAdvisor, so it occasionally includes snippets from member's reviews, and it makes a point of highlighting the pros and cons. Fodor's writes of the same hotel, which it calls "the best hotel for experiencing luxury, Cancun style" both its top-notch service and huge spa and under "Cons:" says "lacks the festive mood of other hotels along the strip; breakfast buffet costs $25."
In general, both books gave me a sense of what to expect: gorgeous beaches, lots of tequila-based drinks (another tamarind margarita please!), and several excellent restaurants. They share the same weaknesses, though, in that neither gave me any sense of just how much everyone in Cancun is trying to sell you tourist excursions, or the overwhelming number of people at Chichen Itza who are lined up to sell souvenirs. I would have appreciated a section on "how to get the best deal on tours and excursions" but neither book was helpful in that regard.
The Fodor's book has a few sections that do help quite a bit. For example, it includes a comparison chart for Cancun spas, including the prices of facials and body treatments, as well as the availability of couples treatments or outdoor treatments. It also has a 7-page section on tequila that describes everything from its harvest and distillation to popular brands. Frommer's has good introductory sections for each region, explaining how the streets are numbered for instance, and it also recommends websites for each area.
You probably don't need more than one of these books, since so much of their information is duplicated. But it's a tough choice, since both are good guidebooks. I lean towards the Fodor's guide slightly because of the comparison charts and an effort to point out hotel disadvantages, but you won't go wrong with either of these. If you find one book cheaper than the other, that's as good a dividing line as any.