The value of Bitcoin and Bitcoin cash post fork should roughly equal the existing value now, you can't create value out of thin air. However I see the total market cap of both dropping below the existing market cap as some level of fear and uncertainty takes hold. As long as Bitcoin comes out on top and Bitcoin cash acts like eth classic, BTC will then go back to appreciating.

So there is merit to selling and rebuying post fork. This also protect s in the crazy case of btc cash gaining traction, confusion setting in, money fleeing to eth and the flippening happening (note I put this at less than 1% chance).