Superman pro-reg? That'd be a huge, and bold, move. The outcome would be far greater than Batman's outing in Kingdom Come. I imagine the JLA and JSA would be torn apart. Half of which would join Superman and Lex ( ) while the rest joined or formed other teams. Robin would withdraw from the Titans. Cyborg would join. Bats, after trying to beat Luthor to a pulp, would take over the Outsiders. They'd probably be hunted by Lex's newly formed Justice League. Superman and Batman would never trust each other again...Bats would be floored that Superman joined with Lex's side.

I'd be with Batman.