Both are necessary and okay to do. There's no advantage too much over the other except with the marketing side, IMO. IEO can gather more people to have the "fundraising" part and is already going to be listed to the certain exchange they are holding at, that's a sure thing. For listing a token to exchange, it requires a lot of work to be recognized and accepted.

Are you planning to list your own? Or do you have a token OP?