^ I'm only doing HTML and CSS (and a very few jQuery) so my opinion is basically on my personal usage. I don't know which of those have better code or which one performs better.
No.1 reason why I don't like jQuery UI is that it adds/manipulate DOM elements and classes on load. For example, when I want to have Overlay/Modal, it adds elements for close button with lots of classes. It even adds useless DIV elements. Whereas with jQuery Tools, you will manually write the HTML elements that you want to become the Overlay - no useless markups that gets generated.
No. 2 is styling. I want complete control of CSS. In jQuery UI even if you don't use a theme, it seems that you are forced to use its generated classes - well, why use another class name if there's one already?
Also, jQuery Tools is a smaller in size.
In summary, I want total control of my mark-up and CSS
jQuery UI size is really too big. When I see how developers use it with Wordpress, it's crazy. Sometimes they could use some basic jQuery code, but no... they use jQuery UI instead which makes the pages load very slowly.
As you can see in the last link, as ForoStyle said, there are serious compatibly problems with the last version of jQuery and jQuery Tools. Knowing that XenForo is using a lot the jQuery tools, it will prevent it to update to the last version of jQuery. Which means the first question of ForoStyle is a really good one.
When you download jQuery UI, yes the file size is a little larger. But that is because it's not optimized (minified) from their site.... Once you optimize it, its actually smaller. This being that you only use the needed libraries .... There is no need to download or use them all.
Total side note, but hopefully it will help someone that runs across the same issue. I have one function that needs the draggable() method of jQuery UI (I'd be happy to do it with jQuery Tools, but they don't have functions for dragging)... Specifically my drag and drop thread movement system for mods/admins.
Long story short is I updated my end from jQuery UI 1.8, and something changed in jQuery UI 1.9.0 (and higher) that breaks jQuery Tool's tooltips (just by loading the jQuery UI script... not even doing anything with it). It turns out they have some functions in the same namespace as jQuery Tools.
So if you *do* need a load jQuery UI, you need to load it before xenforo.js (which is where jQuery Tools resides) so methods like tooltip functionality don't get overwritten by jQuery UI.