It would be more reasonable to buy coins after they appear on stock exchanges, since it is likely that their value will be several times lower than the price at the ICO stage. But first, if everyone expects a coin on exchanges and does not support the project at the ICO stage, then it will not collect the required amount of investments, and secondly, from some projects the value of coins after appearing on the exchanges increases several times, and then you will already miss the opportunity purchase them at a bargain price. For me personally this is a very difficult choice.