With Ubuntu 12.04, the performance of Unity on my Thinkpad X200 (Core 2 Duo, 8GB RAM) was slow, so I used the Gnome Fallback (which was Gnome 2).
With Ubuntu 14.04, Gnome Fallback was no longer available, so I install Mate, and used that an alternative desktop. It mostly stayed out of the way of Unity, and included a different terminal and file manager. I tried Cinnamon, but that caused so many conflicts that I seem to recall having to practically do a reinstall of Ubuntu. Ubuntu 14.04 with a Mate desktop was snappy and productive.
With Ubuntu 16.04, Unity performance had improved so much that Mate wasn't really necessary. However, the impending move at Ubuntu 18.04 from Unity has caused me to explore alternative desktops. KDE Plasma 5.9 on top of Ubuntu 16.04 caused issues with LightDM. Rather than fight the issues, I moved to Kubuntu 17.04 (with KDE Plasma 5.9) and was really impressed with performance. I've now moved to Kubuntu 17.10 because an advanced setting for an L2TP VPN was fixed with that.
So, if you're looking at coexistence with Gnome 3, my guess is that Mate would causes fewer issues.
If the user is coming from a Windows experience, it's worth paying attention to KDE Plasma after version 5.9, as the development processes on the team have changed drastically over the past 2 years. The desktop team now works primarily on KDE Neon (frequent updates to KDE on an Ubuntu LTS base), and there seems to be enough crossover members so that Kubuntu (at the non-LTS version) gets integrated and updated.
If the user is coming from a Mac experience, I now recommend Deepin Linux, which is a complete desktop environment (centered out of Wuhan, China) built on top of Debian unstable. I had tried Manjaro Deepin, but Arch wasn't playing well with Grub on a triple-boot laptop. A casual Mac user would probably prefer the integration in Deepin Linux, while the hard core developer coming from a Mac might prefer the depth of Arch repositories available via Manjaro Deepin.
This reports on my experiments in 2017. I have a colleague from Finland who is on an assignment in China, so my computer support for her needs to be bulletproof.