If you want something that's capable of being installed by a normal person and not only a professional system mechanic, you have Shimmie.
Otherwise there's no real reason to not use danbooru.
Posted under General
Last I checked, Gelbooru is closed-source (it's also coded in PHP, not Ruby, like Danbooru is), so you wouldn't be able to use it anyway.
So, by default, I guess you should go with Shimmie, then.
Edit: By 'Gelbooru', did you mean a "Danbooru-like" image board? In that case, I'd still recommend using Shimmie.
But if you're technically inclined, and know a bit about system administration and back-end management, I would heartily recommend using Danbooru instead.
Well, seeing as he referred to image board software simply as a "script", I could only assume he's not very technically inclined (sorry if I'm being over-presumptuous, OP). So, meh, I sorta had to recommend Shimmie, since it looks like he probably wouldn't know how to setup or work with the Danbooru software.
Though, for anyone capable out there who would've had a choice, I'd recommend Danbooru over Shimmie. Every time.
Edit: And it also appears I was incorrect about Gelbooru being closed-source, as ltu pointed-out. Sorry.
Updated
Gelbooru has a 0.1.x version released for public use. It is a bit confusing to set up at first, I'll admit, but if you've messed with configuration files before in your lifetime, then it shouldn't be much different. Currently the dev version is the most stable. It is available on the SVN.
I've been told Gelbooru has better performance than Shimmie if only because it doesn't use 100% CPU for each image page load. However, once you reach a certain amount of updates to an image, performance will suffer badly. (Table locking)
You can PM me here if you want help installing it or want the dev version.