It depends on what you're willing to do and what you can provide.
Since you have experience with iguanas i'm not going to ask the obvious (do you know how big it's gonna get/what it's gonna eat/ect)
Here is what I have discovered in my 2yr experience of raising each.
My niles tend to be more active and want to explore whenever they aren't basking or cooling down.
My savannah is more of a "chill" type lizard as is pretty content just hanging around most of the time.
The niles are more unpredictable and are easily excitable/startled where again the savannah is more predictable and harder to startle.
The niles are far more aggressive and "cocky" where the Savannah is more of a defender playing defense more then offense when approached.
Basically everything says the same thing.
The Savannahs are more calm and simple mannered monitors, where the Niles are more "hardcore" beastly mannered monitors.
I have (and still do) taken bites and tail lashes from the Niles whenever they feel uncomfortable or vulnerable and this is expected. Best thing to do is not to put them in that situation. Not so much the Savannah.
Both lizards are a little spastic when babies and slow down with age.
Bottom line is that Niles require more training to be tame because they are fierce natured lizards. If you are willing to put the time and energy into taming a Nile they can become wonderful pets but you always have to remember they have that temperament and require more respect and caution when it comes to handling. I think the naughtiest lizards I have ever handled are baby niles. But that doesn't stop someone who is determined to have a tame pet nile monitor, and with a little work and a lot of handling you too can have a pet dinosaur eating from a spoon.
both of the older guys are very tame and can be carried around like a teddy bear without fear of getting bit. I even take them to music festivals and they don't mind being held or pet by strangers as long as they aren't being hurt or handled aggressively. They just have to trust you and be used to being handled.
hope this helps!
Don't hesitate to ask questions!