Time machine with a second internal disk would be good, especially given the capabilities and overall purpose of time machine.

If you could swing it, I'd do 500gb boot+data disk, 750gb or 1tb time machine disk, and a 500gb external disk to keep a weekly or monthly superduper or carboncopycloner image of the boot disk.

This way, you have both a complete configuration you can go back to, as well as up to the moment updates on your user data.