Ive had *some* experience with both of these (having kept a Nile for the last 2 years and having spent quite a bit of time around a few different Savs).
For the most part, yeah, Niles are the bigger, badder lizard. They compete almost directly with Nile Crocs... you gotta be hardcore if youre the lizard that mucks with Crocs, lol
While both species can get BIG, Niles (because of that extra long, powerful tail) tend to get very long. This can make housing them difficult (any 6'+ lizard requires alot of space, an 8'+ requires A LOT).
Many people say that Niles are more openly aggressive than Savs- this I dont think Id agree on. Absolutely the meanest, most outright hateful animal Ive ever encountered was a 4 yr old Sav that was less than 30" long. He'd been kept in a 20 gal. aquarium his entire life with a filthy water bowl, a half log for hiding and a substrate of sand(though by the time we got there, it was mostly poop) and though fat in the belly, he was REALLY undersized and looked sickly.
My friend bought him from this kid, in hopes of at least giving him a larger home. Never have I seen an animal respond to humans in such a way- this animal hissed on the verge of roaring, I swear to God it was aiming its poop, and the look in its eyes was INTENSE.
My Nile is "assertive" for the most part, not liking to be picked up (I can touch him on the back and on the head or tail, but if i try to pick him up, WATCH OUT! hehe). If I do pick him up, he hisses and scratches and sometimes poops. But not like this Sav did. Even when my Nile fights hard to get away or is really upset, he dosnt act like this thing did.
The animal survived another 10 months or something, gained like 7-8" and then miraculously died.
It just goes to show that every monitor is different- they havedifferent personalities, they have had different experiences, and therefore different perspectives on their world.
Niles as a group, tend to be highly aggressive, but I think individuals can all be turned with time and trust- in the case of Niles, turned towards a calmer and more trusting animal.
If I were you, and I had expereience with reptiles(large and small) but not monitors directly, and had $$ to spend, Id prolly take a look at Ackies. They dont really get big, but in the room youd house a single Adult Nile you can prolly house a breeding group of Ackies (granted, the Acks would cost you about $900 vs the Nile at $20-30). There are tons of monitor species, find what fits you best.
Peach-throats also seem really cool, visually as well a behaviorally (i dont think thats a word...), but again, you need some $$.
Timors seem interesting, though from what I understand of them... well, to quote the person I heard about it from "I was told if I was doing things right, Id never see him... I never see him and food continually disappears, so Im taking that as a good sign".