For the curious, my reasons:
I wanted to stop spending so much time mending & maintaining.
I wished to switch to KDE – it has improved loads over the years and is way better than GNOME was before it went to shit and is way more featureful and nicer to use than XFCE. Gentoo, despite its bleeding edge reputation, is still on the last version of KDE.
The open source drivers for AMD / ATI cards are fairly good now, and do what I need a little better than the official ones and work with newer kernels.
When they work.
Under Gentoo I could never figure it out. Probably not Gentoo’s fault, they also work badly under OpenSUSE and Fedora too. I can’t use both screens at once, trying makes one lose the signal but the desktop is still using it. Disabling the second screen doesn’t make the first one come back on, just disables the second screen with the first still blank, this persists through a restart. On top of this daft behaviour, Fedora even only displays the login on your first screen – but first screen isn’t defined – in my case it happens to be the off display. Useful. In Ubuntu all that works just dandy.
Both Fedora and OpenSUSE were great choices, but they are also both interested in open source purity – they don’t come with even slightly non-open stuff. Both come with nerfed FFMPEGs, and are thus useless. Fedora has third party stuff, but like I say, couldn’t get the screen working. I didn’t find similar for OpenSUSE, which is a shame because I loved everything else about OpenSUSE – the rolling version was completely stable, yet joyously up to date, much more so than Gentoo, it even has FFMPEG by default over LibAV, which Gentoo came around to as well recently.
Here’s a good comparison point. I use KDEnlive for my non-linear video editing needs. This uses mlt. Older versions of mlt add green tints to things, but that was fixed. OpenSUSE has the most up to date (March 2015) version. Gentoo’s is from June 2, 2013. Ubuntu’s is a little older than OpenSUSE and not the latest, but they have the source package for the most recent, and it’s surprisingly easy to compile and install from their source deb.
It feels pretty strange abandoning Gentoo because of its out of date packages as that’s supposed to be its great strength.
So Kubuntu because:
Latest KDE5 stuff.
Open source graphics drivers that do everything sanely.
Easy to maintain.
Not so terrified of non-completely free stuff that they cripple FFMPEG.
Up to date in general (but especially KDE)
I can put up with having to jump an extra hoop for latest FFMPEG and libAV by default because I have the above.