There is no relationship between the two libraries except for the fact that they are both built on top of jQuery.
In my experience, both libraries work very well for the set of feature that they provide. You need to have a look at the feature set on the home page and decide on what you need, then make your choice.
You can use both of them at the same time if you need to, so to answer your comment on another answer, using the drag & drop from jQueryUI and tooltip from jQuery Tools is definitely an option. If you can use only one library it's probably best to though. Your code will be clearer (as every library is coded with a different style) and your users will have less javascript files to download when they visit your website.
As for documentation, I would expect you to find more documentation on jQueryUI, as it is used by many people all over the place (I don't have numbers or anything, but my guess is that jQueryUI is used a lot more than jQuery tools). The documentation on the jQueryUI website is amazing and provides examples for every component.