I like them both. I think of the Biblical and Qur'anic myths of Abraham (Ibrahim) and Ishmael (Ismail). Both flawed but ultimately good men trying to make sense out of chaos. Of the names themselves, I prefer Ismail slightly, but they're both good.
Ibrahim is my favourite. I think I like it because there's no doubt of the pronounciation. I'm never sure how to say Ismail. I could look it up, though, right? :-)