http://jqueryvsmootools.com/
Written by a MooTools person, but seems mostly accurate. (I think MooTools might even be a tad more JavaScript-ier than jQuery, but only just, *if* at all--you'll get other opinions about that).
One thing from that article that I think sums up my feelings about the two is this:
jQuery Makes the DOM More Fun. MooTools Makes JavaScript Itself More Fun.
This isn't a value judgement--I think the two libraries actually have different purposes, although there's clearly some functional overlap.
Both jQuery and MooTools are really good libraries, though, just with different underlying purposes and philosophies. The article, while definitely written with an opinion, is a great resource for understanding the differences, and will likely teach something about JavaScript and the JavaScript thought process. Good read.