Will you have a car and are you planning to stay in one of these places as your base (the whole time) or are you asking which of these to visit?
Also, where are you flying in?
Mondello will likely be least touristy and will have mainly Italians. As other have said, there is plenty of dining and some hiking.
Cefalu’ will have more tourists and has hiking and a great church.
One option is to stay at Mondello or Cefalu’ and then visit the other. From Mondello, you could also day trip, easily to Palermo and Monreale. Really you could also do this from Cefalu’, too, with a little more effort.
Taormina is nice also but probably the most touristy. It does have nice views and is not far from Mt. Etna.
Depending on where you are flying in, your travel preferences, and whether you are comfortable moving around, you could do both. 4 nights near Palermo and 3 near Taormina. This, without knowing where you are flying in/out and if you have any other plans.
Personally, I like the western part of the island best and I like the chaos of Palermo so I am a bit biased.