I have never owned a monitor but I have a 4 year old Argentine tegu that I wouldn't trade for ANYTHING! He is awesome and has a great personality. He is nothing at all like a python (in regards to caging, interacting, feeding, lighting/heating, etc. ---he requires a lot more attention) but in my opinion he is a lot of fun and has an individual personality (he has the demeanor of a cat). AND what does make raising Argentine tegus easier to raise is that they hibernate for about 4-5 months out of the year.
Overall large lizards (in my experience) are VERY rewarding, but they do require a large commitment compared with any other reptile I have owned. And make sure to do LOTS and LOTS of research before diving in. I researched tegus for about 6 months before ordering one from Bobby at Varnyard (highly recommend) before it was born and waited about 4 months to get it.