Which would be easier to use to help lift the lower leg part of an exoskeletal leg? And are there any suggestions of the exact cylinder or actuator to use, I was thinking of something about the size of a tibia.
Go with electricity (and lithium batteries). If you have to drag a complete hydraulics system around, the pros of an exo' sort of vanishes.
To get even slightly useable suggestions, you need to present a bit more data (how much force, speed, load etc. is needed).
You might wanna look at this page
and you can find several research papers on the subject (like starting here
or perhaps try PubMed).