The main difference between ICO and IEO is that during an IEO, exchanges actively participate in the fundraising process, ensuring that only the most promising and viable projects are listed. As for me, IEO better than ICO for following reason.
The other difference is the agressive marketing strategy of IDOs providers. In fact to be able to participate in IDOs you need to buy and hold their native tokens. Which means that the number of people willing to do so will be reduced. Now when you have a token for sale and a restricted number of people (knowing that theses people have already spent around 500 $ to get in) the price will go up (yes IDOs token price is not fixed)because of demand increase. Then when the same token get listed in exchanges thus a more important number of people concerned if the project gets ignored then the price will drop. result you would have bought an expansive token during IDO because of big demand and then loos your money once it get listed.