If you're thinking either V. exanthematicus or T. merianae, then go with the tegu. Both require a lot of knowledge and experience to maintain properly; but for a beginner (or even someone with lots of experience) you will find a tegu more rewarding than a savannah.

Ackies are great lizards as well, but if you are looking for something larger, they won't satisfy that desire. Not in the least.

All savs are wild caught. You can get captive bred tegus from a number of places in the States.

I recommend Varnyard.

Chris