It's really hard to compare the two. A jpg, which is short for JPEG stands for Joint Photographic Experts Group and, as its name implies, is a format for photographs. It uses compression, actually, lossy compression to compress photographic images for computers. So, you certainly wouldn't save documents in a jpg format. They are fairly small, though, due to the compresssion.
Again, I think it's a no brainer. Save everything as a PDF file. Just add disk space as necessary. RAM and hard drives are dirt cheap nowadays, so there is no reason not to get larger drives.
