To learn YUI or JQuery?

I have been reading up on JavaScript libraries with a view to using one of them for a few little projects I have in mind.  The two main ones, YUI and JQuery both seem very nice and are great improvements on programming in bare JavaScript.  Originally, I was tending towards YUI because it was the first one I came across, and because of the extensive documentation on the Yahoo! web-site.  But then I did a search for job vacancies mentioning these two libraries,  There are around 20 times as many UK vacancies mentioning JQuery than those mentioning YUI.  That settles it, at least for the time being, I am going to learn JQuery.