The advice is to take your BTC out of exchanges as exchanges will opt for one or the other. After the fork you should be able to exchanges that either do all the forks or different exchange for each fork and convert to each fork. Eg. you if you have 1BTC that will equal 1BTC and 1BCC after the fork.
The values for each of these are yet to be determined
Bittrex has stated they will support whatever happens, and you'll have BCC worth whatever BTC you had on the exchange on July 31st-August 1st.
Same is true for Kraken http://blog.kraken.com/post/1150/bitcoin-cash-and-a-critical-alert-for-bitcoin-margin-traders/