If weightloss is a primary concern which is better...MC or water fast?

The Oxypowder has never been part of the MC. I'm glad that you are fine but there are several possibilities that you can do harm doing the OP on the MC. First you could realease more toxins that the system can handle all at once and make yourself sick. Two if you are doing both the OP and the SWF (Salt-Water-Flush) there's also a possibility for dehydration so  make sure you are drinking lots of water thoughout the day and third very likely you are interfeering with the cleansing action of the lemon and cayenne and because of the relatively agressive nature of the OP you are probalbly depriving your body of a lot of the nutrition provided by the lemon and the syrup since that is >all< the nutrition you are taking in.

The Master-Cleanse is a very simple, elegant and very efficient way to cleanse your system just the way it was written. More is not necesarily better. The OP can and should be done while eating regular.