So the only real difference is Lieberman is more pro-abortion?
No, that is not the only real difference. If you compare their ratings by various interest groups, you will find that Lieberman is much more liberal on one issue after another. As I've said, I don't like McCain and I hope the Republicans nominate one of the many good solid conservatives in the party. But if it comes down to a choice between McCain and Lieberman, I could not vote for the more liberal candidate.
You can compare for yourself at the following sites.
http://www.vote-smart.org/issue_rating_category.php?can_id=S0061103
http://www.vote-smart.org/issue_rating_category.php?can_id=S0141103