Buy later on exchange is the best choice because now many ico are failed. Many ico don't enter the market. So instead of losing you better buy coins in the market.
With certainty it is best to buy when that coin is listed on good exchanges. I think this will be much safer and people will have more new choices in investing. ICO projects have been underdeveloped and lack of trust from the community in the past, which has given the IEO a chance and it has helped the market to have a very good recovery in 2019. Personally, I will only invest early when people know about the project and the low purchase rate will help me a lot in making high profits.