I know a lot about bengals (I have a 12lbs female bengal) and some about savannahs (I know a guy who has one) and basically if you want the really big Savannahs you need to get the early generation ones who are still very close to the Serval.
The early generation Savannahs can be hard to keep, I mean if you're willing to put in the work and take care of them properly and their personality suits you it's really rewarding but there are a lot of ifs there.
They are VERY active and powerful and will jump up to 7 feet in the air to go after their prey. They love being high up and jumping so your fridge, kitchen cabinets, tops of your doors etc. will be part of the day to day terrain for these cats (this happens quite a bit with other cats as well though)
The early gen cats have very sensitive digestive systems and can only cope with the very best commercial cat foods and some need to be on a specially prepared raw meat and organs and other supplements diet or they'll have constant diarreah and won't thrive. This is also true of Bengals.
The Savannah I know of is an F3 and he has a special room in his owners house where he's locked in when he's unsupervised (the room is large and filled with climbing trees and cat toys etc.) because they don't dare let him be on his own in the general house because he gets into everything.
Average sized cats can be bad enough when they get into things but when it's a that much larger cat the issues are worse. Rather than just throwing books to the floor, the entire bookcase can topple etc.
These cats are definately not lapcats. Although it varies with each cat of course. Basically Savannahs are not gentle giants, they're very powerful very active giants that need a productive outlet for all that bundled up energy or they'll become destructive.
The lower gen cats are more like domestic cats but they also look more like domestic kitties and are a lot smaller and because the breed is still so new the variability is quite large so you can get very different looking cats who are all genetically Savannah cats.
Bengals have been bred for longer so even the cats who are very far away from the original wild cat have a very distinctive spotted look. They are smaller though but very compact and muscular. So a Savannah weighing the same as a Bengal will look bigger.
Also early gen cats (both Savannahs and Bengals) are often ruled by tighter legislation than domestic cats since they're counted as hybrids and are illegal in some states as well and it can be very tricky moving them between countries too.
If you just want a large cat I'd probably go for a well bred Maine Coon, if you want to do a lot more research and learning and you think you're a good match for a Savannah, well good luck but keep us posted no matter what you do