When comparing Gallium OS vs Haiku, the Slant community recommends Haiku for most people. In the question“What is the best desktop OS?” Haiku is ranked 29th while Gallium OS is ranked 34th.
Ranked in these QuestionsQuestion Ranking
Pro Chromebook hardware support baked in
Many things work out of the box that do not with other distros, such as touchscreens.
Pro Chromebook specific OS
Gallium OS is engineered with Chromebook hardware in mind to make the best use of Chromebook CPU's, limited RAM, and storage limitations.
Pro Easy to install
Using chrx on Chromebook there are only two or three straightforward steps to get Gallium OS running.
Pro Dual boot
Installation option to dual-boot GalliumOS alongside ChromeOS.
Pro Very Fast
Pro Beta has been released
After about 6 years since the alpha version, beta has been released on Fri, 2018-09-28. Check here for release notes.
Pro Only need 512mb ram
Con Not the best looking UI
It's not the best in terms of UI but you can always install themes
Con No one uses it
It's a very niche OS that no one uses.
Con Small community
It is important when developing to be familiar with tools that other developers use. You can make any utility in any language you feel like, but if it's in an esoteric language that no one can read targeting a small platform that no one uses, then it was just something you did as a hobbyist, not as a developer.
This is not to say that Haiku isn't a great operating system to hack around on. Just don't delude yourself into thinking you're doing it to get familiar with tools that you need to know to be a better developer.
It's still in beta and quite unstable. Making it unsuitable for developing applications of any kind.