Our visa was due to expire on the 18th January 2011 and it was now the 11th January. We knew that we had an over night journey to get to Nah Trang and from there a 3-4hr journey to either Dalat or Mui Ne, plus travelling back, so time really wasn't on our side.
I really wanted to visit both places but we had to decide on one as we just didn't have enough time to do both. We had heard good things about Dalat but no one we had met had been to Mui Ne. After weighing up the pros and cons we chose Mui Ne.
The only reason we picked Mui Ne was because of the wet and cold weather we had experienced for the past 3 weeks. Dalat is in the mountains and gets quite cold (think almost Sapa cold) and Mui Ne was right on the beach, experienced very little rain and would be of a much warmer climate.
My advice would be to visit both, if possible. Even though i have never been to Dalat, everyone that we met had said it was worth visiting.