the price dropped because the news came out a while ago (a couple of weeks now) and when the news comes out the pump starts and price reaches the high values not when the market is created. that is how the news works. there is a saying that goes "buy the news sell the rumor" or something like that
in any case, addition of bitcoin cash to BitStamp is good for its future pumps which can be bigger. if you want to invest you must do it at the bottom after the dumps ended and the bubble of it was popped.
but in case of BCH because of manipulations you can not tell when the bubble starts and ends. instead you have to watch the market for the pump indicators such as increase in the volume.