Yes, you can get a lot more grunt out of a lot less space with Li-on batteries - the big issue is the charging regime which, at the time that I investigated, is not entirely straightforward. GE Rik of this parish has quite a lot of info about Li-on technology as he uses them exclusively
Now to find a link to his blog spot - I may be some while
Peckforton Light Railway: Quick introduction to the iMax B6 charger
Every one of those blue Li-on batteries I buy comes with it's own charger. The chargers are of the wall wart type. The other Li-on batteries I use are of the built up cell variety. they use a smart charger that I purchased separately from All Battery dot com. These batteries are the 14.8 volt type and power my larger locos and trailing cars.
I've been very happy with the Li-on technology. It's much more technical than I and smarter too..........I'm sure that I am not telling you anything you don't already know. If you find Rik's blog spot, let us know. I seem to recall it also.