I have lived in Coppell for over five years and love it. Coppell is about 20 miles to downtown Dallas, which is not a bad commute (as I used to drive it). In my opinion, everything you need can be found in or closely around Coppell as it is centrally located with access to four major freeways. Unlike Coppell, getting around town in Flower Mound is impossible with virtually no quick access to any major freeways. It is true that Coppell's residential areas are built out and are no longer in the development stage (except for some higher-end homes currently being built i.e., $800K +), but this only means the housing values will continue to increase in Coppell. Also, Coppell still has a somewhat small-town feeling where you can actually run into people you know while out and about and everyone is friendly. In my experience, Flower Mound is becoming so congested with both commercial and residential development that it does not have that same feeling. Finally, the Coppell schools are exemplary, which in my opinion are also better than most (if not all) Flower Mound schools...the Coppell ISD is presently addressing the overcrowding issue at the high school (ie., building another highschool or converting a pre-existing school to another high school), which should hopefully be resolved within the next two years (from information I have read and heard in the community).
Overall, it really depends on what you are looking for in a community. I believe both cities are family-friendly and have comparable resources for families (parks, playgrounds, excellent libraries, etc.)For my family, Coppell is the only place to reside! I hope this helps you.