Correct East is flat (old farm land), West starts to get more hills / mountains.
The areas I mentioned are very active, nice light rail access, good community that is well designed and has lots of things to do such as riding a bike around Chatfield, Deer Creek, hiking boating, biking, many free rec centers.
Parker is a little more rural with less options, of course you can always drive west for the mountains north west for downtown Denver / Cherry Creek. It is less expensive to live in Parker due to the fact that it is further east and away from major things in the area.
Do not discount the Light Rail access as it is a great way to commute to downtown Denver for work or play. My wife used it to get to work from Highlands Ranch daily and we would take it to the sporting events, conventions as they are all downtown and parking can be an expensive PITA.
I do not mind having a short commute ~30 mins to work if I can live in a place that offers a bunch things to do in a nice safe community VS a 10 min commute to work and having to driving 30 + mins when I want to do something else.
It really just depends on what they want.