I personally have done water fasting, juice fasting as well as the MC. The Master-Cleanse worked for me, for the amount of days I was on it, the 1st time, I did 12 days, and lost 21lbs, and a couple months ago, i did 7 days and lost 15lbs.
I did a 35 day water AND juice fast and lost 41lbs. Obviously its going to come down to your own body, but I dont feel that my metabolism slowed because of the MC. I think to be successful at any kind of fast, all relies on your mental strength. (which mine isnt so great right now lol) You can to decide whats best for yourself and your own strengths are. If you can stand complete water, then awesome, but for me, I decided that doing doing straight water, and then maybe 1 glass of juice every 3 days ago helped me stick to it for 35 days when I really wasnt even planning on fasting.