title_body: Elder flower champagne best to use natural yeast or add a wine yeast?
upvoted_answer: I will try to answer your questions to the best of my ability.
  
  Your description of your earlier wine makes me question you: 
  Have you used a hydrometer to calculate the alcohol percentage of your earlier wines? 
  
  If your wine tasted sweet after fermentation, it means your wine wasn't finished fermenting. If you bottle the wine and let it stay in room temperature with unfermented wine, then you may create bottle bombs.
  
  Comparison between wild yeast and adding wine yeast:
  
  
  Duration of Fermentation: Naturally occurring wild yeast will use longer time than wine yeast. Adding Wine yeast is a lot faster, and will substantially reduce the fermentation time.
  Taste: The taste is subjective, where wine yeast may create a safer and cleaner taste, meanwhile wild yeast and other bacteria may produce 'unwanted flavors' or 'wanted flavors'. Everything depends on what you want. I don't know enough about the specific tastes to tell you here.
  Alcohol percentage: I recommend using a hydrometer before and after fermentation to calculate the alcohol percentage of your wine. Adding wine yeast will eat up all the sugar in your wine very fast. 1-2 weeks, while wild yeast may use several months. Everything depends on how much yeast is present in the fermentation, temperature and how much sugar is in the unfermented wine.
  
  
  Wild yeast takes time to grow into a culture which can eat up all the sugar, where wine yeast usually already has ~1 billion cells in the package.