There shouldn't be a significant difference in performance that I can
see.

By and large, though, use of Observable/Observer is very rare.  Because
it requires that you subclass the Observable class, it's not very clean,
easy to retro-fit into existing code, nor easy to refactor after it's
done.  For the most part, modern projects use the event listener idiom
instead, by declaring their own interfaces with callback methods.  See
Swing for plenty of examples.

Requiring a single listener is actually considered proper for the event
listener idiom, so you're in luck... if you are still worried about
keeping a list of listeners, you don't have to.  Just call your method
setXXXListener instead of addXXXListener.