Introduction
I've been using jQuery regularly for my new job now for about 3 weeks or so. I've used the selectors, event handling, short cuts/handlers, animations, plugins etc. Nothing too fancy, but in general, a descent exposure. I'd like to compare it with MooTools. And while I do rock a MooTools skin on my phone, I'm going to try and remain as objective as subjectively possible. I'm going to try and stay positive about the differences and give use-cases for using either.
Gut Reaction
jQuery is fast to learn, easy to use, and passes the 'thinking' to the framework. It allows you to do incredibly powerful client side stuff without knowing too much of the inner workings, worrying about being specific and explicit about your syntax or organization, and most importantly lets you get this stuff done quickly.
MooTools on the other hand requires you to be explicit. You want to create a node with 4 children and easy of them have a bunch of inline attributes? Cool, do it manually, instead of specifying a string of html which the engine will parse and DOM-inize for you (jQuery can do this). Most prominent for me: jQuery is very smart and very easy to use, but lacks a certain elegance due it's lack of explicit control; MooTools requires more work and overhead to get something done, but is very organized and beyond the learning curve, more meaningful.
Example
To create a series of nodes in jQuery, it's as easy as this:
var html = '<div id="div-one"><span id="span-one">yo yo</span></div>';
$(html).prependTo($('#random-node'));
In Moo, it'd be something like this:
var first = new Element('div',{'id':'div-one'});
var two = new Element('span',{'id':'span-one'});
first.adopt(two);
$('body').shift().adopt(first);
One is easier/more implicit than the other. However the other is much more explicit, and easier to follow for a foreigner who isn't too familiar with the jQuery syntax, rules and behaviours.
Optimization/Speed
I don't know too much about jQuery's speed. I believe that at this point we've got to the point where an extra 10 milliseconds isn't a do or die or decision maker for what framework/library to use. But with that in mind, one of the great things for Moo is the ability to compartmentalize only what is needed for client side behaviour. Everything is kept as a separate stand alone, window based class, where as jQuery seems to throw in a lot of other code into one batch which is mutually dependent.
This goes back to my gut reaction though. For a new comer, I don't want to try to determine what classes/objects I'll need. I want a script tag that has almost everything that I need. No playing with downloader apps to pick and choose what my project will need. MooTools therefore introduces a higher level of (initial) complexity, but brings with it greater than control, and again, explicitness.
Community
Up until this point, I've more or less tried giving opinions for and against both libraries, but in this category (Community), there is a winner hands down: jQuery. I don't really know what the reason for this is. I would say that jQuery has been more organized in their community/developer evangelism, which is completely true, but I don't think that's the real reason.
While MooTools is still trying to get it's community plugin system up and rung
(MooForge, I believe), I think a core reason it's the direct and simplicity of
jQuery. I think it was able to garner such a strong following because the
barrier to entry is to incredibly tiny. I don't need to understand JavaScript
syntax, programming syntax, or the inner workings of client side development.
The very syntax that I love about Moo (e.g. new
Element('div').addClass('oliver').addEvent...) is also probably what turns
people off. The fact that I need to be so direct about what I want done is
something that many people probably don't want to deal with.
Conclusion
But away from my tangent, the jQuery community is amazing. So many plugins, organized horizontally and vertically by jQuery themselves. So many resources to learn the syntax, deconstruct plugins to understand the code, etc. While my obsessive compulsiveness when it comes to coding syntax and practices would never allow me to fully embrace and adopt jQuery personally and independently of where I work full time, I can understand fully it's benefits. I'll always evangelize MooTools whenever I can, but after this exposure to jQuery, I'll understand where it's most useful and for whom MooTools would really be welcome to.