My impression from the stuff I have read so far is that Brasov is a good center of operations if you have your own transportation to make daily excursions around the area. Sighisoara is a smaller much more compact city with lots of stuff of historical and cultural interest even within walking distance if you stay in or around the "Citadel". So although it is a longer train ride, it might be a better place to stay if you are not going to rent your own transportation.

But I'd be interested to hear your impressions. We'll leave for there soon.