Selling bitcoins before the split would be a wise decision any way you look at it. It is almost a certain that price will crash after the split (even if temporarily) or even before it (if it is really going to happen, due to insiders cashing out), so the risk is not worth it. If you don't know which camp to stick to, you can buy back both coins at cheaper price, and thus you would be better off overall than if you decided just to hold. I guess I won't have to explain where exactly your logic fails. It should be straightforward
There's an argument to be made that there could be a BTC supply shortage on exchanges prior to August 1st, due to a larger percentage of people pulling their bitcoins off into wallets they control. If that happens, but demand remains constant, you could see the price go higher. I'm currently bearish, but there might be an opportunity there for people willing to take the risk of keeping their bitcoins for sale on the exchanges nearer to August 1st.