In my opinion, basically, to buy during ICO is still the best choice, with the aim of getting a cheaper price of coins. But, of course, this will have a bigger risk than you buy coins when they have listed on the market / exchange. Yeah, the project of the ICO coins maybe scam, cancel ICO, choose a bad exchange for listing the coins, coin prices very low after listing on an exchange, or etc. Perhaps for those reasons, investors prefer to buy the coins once listed on the market / exchange.