I wonder whether that would destroy the working version of Inkscape I currently have (0.48)
Yes, it would (unless you install in a container/VM/chroot environment). Keep a copy of your current preferences file, too, in case you want to uninstall 0.91 and reinstall 0.48 - I don't know if they would be backwards compatible.
Look into the release notes for a list of changes:
http://wiki.inkscape.org/wiki/index.php ... notes/0.91Basic functionality is the same (if you use 0.48.5, which is a very stable / very low-bug release, then stability is worse / bug amount is higher in 0.91, imho), but there are also some fun new features (PowerStroke, arrange on a circle, measure, symbol dialog, keyboard shortcut editor,... and bug fixes) that make me prefer 0.91.