I'd rather buy coins on the stock exchange, because it means that the ICO at least went to the stock exchange, and did not rot at the level of collecting money.
Lots scams end up on exchanges, it is never good to blindly trust coins just because you can find them on exchange. Much better way is to research project and to see if development is active.
If developers are not doing anything and yet their coin is on exchange, all is clear. Only thing they will launch is news until their coin reach certain price and then they will dump it.