I love monitors but the Niles you really have to work with. They just seem to always be stand off'ish no matter how much you work with them. You just really have to take your time with a Nile. The Asia water monitor seem to be easier to tame and work with. They just get a little larger. But either way your going to have your hands full and have t build a huge enclosure for an adult. Niles you can normally pick up cheaper then an Asian since more people would rather work with the Asian.
Have you kept monitors before?
I plan to pick up an Argus here very soon and maybe a green or blue tree monitor.
YOu really need to have experience with monitors to keep one of these guys. They get huge and are generally mean. If you are really set on getting a big monitor like them then go for the water monitor. They look very similar, have similar requirements and are generally much more personable.
Well ive still got bloody cuts on my hands from re-capturing an escaped 2 footer a few days ago.....Beautiful lizards but not for the inexperienced.Not only will they bite down hard,they death-roll like a croc!
You may want to consider a tegu (I personally prefer columbian black and white tegus). They reach impressive but not unmanagable sizes and are docile and personable.
if you go with a tegu, get an argentine, columbians are generally more aggressive and/or extremely skittish. you'll pay a bit more for an argentine but its worth it.
I wouldn't consider a Nile without having a lot of large monitor experience. Personally I've kept a few medium species (savanna and dumerili) as well as a water monitor, and I'm still not 100% sure I'd take the Nile plunge...and I really like Niles. The little ones are just so skittish, until they grow up and realize they don't have to be afraid because they can fight back. It's possible to earn their trust, but they spook easily and that will undermine all the trust you've earned.