This is quite difficult to answer because both decision can be profitable. We know that ICO in its early sale have many big bonuses which if it's successful can grant us a big profit. What is good in buying coins in exchanges is that we can guarantee that it's already have a value but there's no more bonuses.
Some ico coins are getting cheaper on exchanges. But some of the ico coins are making great price boom and you can't buy them with cheap rates in the market. You always have trouble in this. So never be late for trusted icos.