When comparing Lubuntu vs Chromium OS, the Slant community recommends Lubuntu for most people. In the question“What are the best Linux distributions for an old machine?” Lubuntu is ranked 8th while Chromium OS is ranked 49th. The most important reason people chose Lubuntu is:
Being based on Ubuntu (which itself is based on Debian), there's a good amount of information and support to be found when searching for it, both on Lubuntu specifically and Ubuntu that also applies to Lubuntu. It also means that there are certain security and usability standards enforced and you can use Ubuntu .deb packages.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Part of the Ubuntu family
Being based on Ubuntu (which itself is based on Debian), there's a good amount of information and support to be found when searching for it, both on Lubuntu specifically and Ubuntu that also applies to Lubuntu. It also means that there are certain security and usability standards enforced and you can use Ubuntu .deb packages.
Pro Lightweight
By utilizing minimal software Lubuntu is able to use less resources and power.
Pro Stable
Pro LXDE desktop
Lightweight X11 Desktop Environment performs fast and conserves resources.
Pro Offers a long term support release
Lubuntu offers a long term support release with their 14.04 Trusty Tahr based version.
Pro Uses Openbox as the default window manager
Lubuntu (LXDE) uses the lightweight Openbox window manager to manage its windows. By using Openbox by default LXDE is able to remain lightweight as there is no built in composite manager in Openbox to take up resources.
Pro Safe and requires little to no maintenant
Since you only have the browser, you cannot install applications (outside Chrome Apps which are decently protected by their limited API and the Chrome Web Store). This means no harmful virus, and also nearly no way you can screw your system. Everything will just work.
Pro Minimalistic interface
Chromium's system UI uses as little screen space as possible by combining apps and standard web pages into a minimal tab strip: While existing operating systems have web tabs and native applications in two separate strips, Chromium OS combines these, giving access to everything from one strip. The tab is the equivalent of a desktop application's title bar; the frame containing the tabs is a simple mechanism for managing sets of those applications and pages.
Pro Lightweight
Because Chromium OS is designed for users who spend most of their computer time on the Web, it is intended for use in computers with little local storage and fast boot-up times.
Cons
Con No composite manager
Being a modern desktop many users expect compositing, even on lightweight systems. Without compositing there will be screen tearing, a lot of it. Though there is built in support for users to add their own compositors such as Compton.
Con Long term support not as long as other Ubuntu spins
Lubuntu has not always had LTS support. Luckily they do offer a 3 year LTS now but it is shorter than other Ubuntu spins that receive a full 5 years of support.
Con Clock in panel is broken currently in LXDE
The clock panel applet does not work as expected in the newest versions of LXDE. If the clock is set to not show seconds in the display when waking from suspend the clock will not update until the time actually changes in the applet which means it can take up to a minute for the clock to update when seconds are not shown. The work around to this is to display seconds on the clock which allows it to update after suspend within one second. Sadly the clock does not display seconds correctly as it skips them by one to three at least once a minute, often more.
This is also not the first time there have been problems with the panel clock which clearly illustrates this to be an ongoing problem for the developers.
Con Lacks some window management features
There's no built-in way to snap windows to the sides, re-size them by double clicking the bar, or dragging to the edges.
Con Redundant
There is absolutely no reason to install it, since it uses the same package repositories as Ubuntu and you can transform any Ubuntu install into a Lubuntu install.
Con Google
Possibly harms your privacy.
Con Made for developers
ChromiumOS is mainly made for development, so there exist no official install images and you have to build it from source or use third party images like Arnold's or the waterfall images.
Con No auto-upgrade (unlike Chrome OS)
Chrome OS auto-upgrade the system, but Chromium OS does not. It's possible that they're thinking about adding that feature from the design doc.