I saw your post both here and on the monitor forum. I have one question before I get into anything.
Is this your first reptile? What other experiences have you had?
Now after answering that question,
If this is your first, I would personally say neither. Both animals tend to get obscene on the size scale. Both have the reputation of having possibly nasty dispositions. And while Sand monitors are not disposable in the least green iguanas are. Should down the road you decide to get a green ig, adopt. There are thousands in rescues that will be healthier than the ones I saw at the show.
Care wise on the general whole monitors and iguanas are both considered advanced species and are sold as beginners. Both have the same issues with needing severe heat requirements. Both are in need of large large caging (mind you not being familiar with sand monitors I am generalizing).
It is too soon to determine the temperment of an animal 2 days after purchase. Many do not eat properly until 2 weeks after being aquired. If the animal you bought was a youngster, it will be more snitty. To them everything is a predator.
Check with the folks on the monitor forum regarding care. They will help greatly. You can also stop by the scheduled chats as there is a monitor guy that frequents the room.
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Cindy
PHFaust
Email Cindy