Concerns about what is or is not included in your dean's letter is ultimately a pointless exercise. At the end of the day, residency programs want some kind of information about applicants that includes a rough estimation of how they performed compared to their peers. Even if a school doesn't provide a specific class rank or something that granular in their letter, they are almost certainly going to provide general information about the performance characteristics of the class alongside your own grades, which effectively functions in the same way. This is what was included in my institution's letters - i.e., a histogram of class performance on each clerkship next to my own grade.
Your school is going to provide some kind of comparison data in your dean's letter - either explicitly or implicitly (for example, via carefully coded language). You cannot avoid it, and I would not make this a huge point of worry when trying to choose between schools.