both are important, since the buying point and selling point determine your profit from their difference.
however, if you force me to pick one of them it would be the selling price. that's because when you buy something, you will automatically determine your selling price in mind. thus, you calculate to secure your profit. that is also the reason why you feel secure even when you buy a stock at a relatively high price. in addition, you can buy a dip but you will feel very vexated when you sell it at one point and then the price of that stock is still going up. in conclusion, maximizing your profit is the most optimal way that all people seeks.