Hi AOD! 
I'm not sure of exact scientific anatomy, but they are classified as distinct species, though the taxonomy seems to be constantly under revision. I'll list the species names add I understand them, but depending on who's papers you want to go with, they could be debatable.
The argentine black and white tegu, (Tupinambis merianae), is larger with some males reaching 5', and generally heavier bodied, with the larger, bulkier jowls, especially the males. They are also more omnivorous. As well as generally being calmer animals, a bit more dog like if you will.
The columbian tegu, Tupinambis teguixin, can be either black and white, or more golden. They are smaller rarely exceeding 3', and are almost strictly carnivores. They are much more active critters, and the best way to describe them would be more monitor like. Though they can be very tame. My girl Nereida has a thread or two on here, she's very sweet. They stay leaner, and seem more of a tropical type lizard to me. I keep a large water dish in with the one I have, and she swims quite a bit.
There's also the red tegu, Tupinambis rufescens, which are very cool, and really the only larger lizard I want. They are similar to the argentines.
Are you thinking of getting one? And how's the little beardie?
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