I did both in the same trip, with Munich in between. Loved it! I think one way to not get burned out by "city city city" is to not stay in a hotel that is in the heart of tourism. So in Prague, I stayed in an apartment on Spalena (Spalena Street?), which is a 10 minute walk or so away from the tourist center. In Paris, I stayed in an apartment on Rue St. Dominique, which was not touristy at all. This really helped with that frenzied feeling you can get when being amidst throngs of tourists. And of course, as others say, mix in a couple of day trips to the countryside and it should be a very nice trip for you! Have a fabulous time, no matter what you decide. I'm jealous!