This will be a good time to sell-- bitcoin is going down and people are just dumping on it. Meanwhile on the other side, people are getting the scraps of bitcoin that they can lay their hands on hoping that there will be a next bull run and make themselves rich. but if you are going to study the trend for this year, it is not looking good for the upcoming years for BTC. New coins are emerging and soon BCH will overtake BTC.
Bitcoin cash is one of the coins that is crashing the hardest, I really do not know how the coin that was forked is behaving because I really do not follow bitcoin cash that much but the chances that is going to overtake bitcoin are incredibly low, this is a good time to buy bitcoin or to hold it but not to sell it unless you love to lose money because that is what it's going to happen if you sell.
It's just the influence of the market on how Bitcoin really drop and i believe it is due to most people are pulling out some of their Bitcoin in order to switch to Bitcoin Cash. And it does create a wide panic in the market that tends to panic selling most of their BTC that they were holding.
But i don't believe BCH would just take over the top rank of BTC in such manners, the new guy(altcoin) wouldn't fit in to the community.