you anwser it right , the differnece is how they market the project . IEO mostly success because they become partner of that exchange where they offer there token.
IEO mostly succeed because of the exchange site so basically that depends on the site wherein you launch the IEO. So therefore, not because you do an IEO then it will be a success.
You should choose a good site however it will really cost you a fortune. While some offers free listing to their site but don't expect a sold out.