I highly recommend panther chameleons! We've had our first forray into them just this past year, picked one up at NARBC back in Feb 2010. He's been an absolute blast, and EVERYTHING about them is just cool.
Rainbow colors, disjointed eye movement, and of course predatory tongue grabbing is all pretty darn unique.
Panther chameleon care is the most over-stated in difficulty, while green tree monitors IMO are often under-stated in difficulty. The biggest thing for both that can make all the difference in their health is a varied diet.
I'm not a fan of keeping green tree monitors. As beautiful as they are, they're just not as confident and outgoing a species as many other monitor or large lizard species. What kind of tegu are you thinking of holding off on? I'd love a big varnyard giant B&W someday.