There are several factors at play. First is that BCH is a hard fork, and thus miners need to actively choose to mine BCH instead of BTC. Exchanges need to add support for BCH. You can also look at hash rates, which show how much mining power is behind either coin. You can also look at transaction rates and exchange rates.
Despite all this, market forces may drive one or the other coin up or down over the next days and months depending on how important the differing features are to the community. It could also turn out that there is a market for both feature sets and they both do well.
By default people that don't change to a BCC client will follow the legacy chain, so if BCC is going to be dominant it will take a while to find out most likely. The price is likely to be very volatile while the market figures out which it wants to follow more.
