Juice is very supportive nutritionally and allows you to continue your regular schedule and activities. I have fasted on juice 60 days and done my regular gym routine every fasting day, barring detox crises...
There are a couple options for juice fasting. You can get a juicer and drink 3-6 cups fresh juice daily, along with lots of water (always following a colon cleansing routine every fasting day, as you would on any fast). You can check out the Master Cleanse, a mono juice fast. You drink only one juice, a combination of fresh lemon juice, grade B maple syrup and cayenne pepper that tastes like... lemonade! A big advantage to The Master Cleanse is low maintenance. You can make up a batch of lemonade with much less effort than shopping, cleaning, prepping and juicing fruits and veggies. The Master Cleanse is also very supportive, and you can continue your regular schedule.
Water is a "deeper cleanse", which unless you have done enough cleansing preivously to prepare yourself for it can mean "deeper detox crises"! Water gives you no nutritional support, and most people need to rest quite a bit during a water fast.
If you are interested in learning more about the Master Cleanse, all the details can be found here at curezone on the very active and supportive Master Cleanse Forum .