It really does vary from coin to coin, after the VOX ICO for example when it hit Bittrex the price dropped and continued to do so, the price now is a shadow of the ICO price, but then.
On the other hand you can have a successful ICO where the price is stable and does increase, it is a risk buying at the ICO price but sometimes this risk does pay off.
I agree, it depends on how much bonus and bounties allocated the ICO gave on its ICO period. If the give a lot, then it is better to buy it on exchange for the dumping of these free coins. But if it has not allocated anything then most probably it is better to have it iin ICO. Because of the fact that this investor will never get out at losing end.