I just build a pc, and put Win 10 on it. But I've got a second SSD onto which I'd like to put Linux, and migrate from mac to Linux as my main computer.

I've done a fair bit of research, and decided that I want KDE, and I really want Plasma 5.

I had also decided that Mint would be the best distribution for stability and learning, but I'm ready to install now, and don't want necessarily want to wait until June for the release of mint 18.

So what are the recommended distros for Plasma 5 at the moment? Thanks for the help.

Here's my opinion, suggestion for you:

Since you've already spent time on Mint (ubuntu based) I would recommend you go with Kubuntu 16.04 now and when KDE Neon (user release edition) makes progress (maybe after about a month) switch to that because Neon will give you the latest and greatest KDE stable releases over time!

Kubuntu has slightly older KDE (Plasma) version 5.5.x while currently 5.6.x is running, they will eventually get 5.6.x on as well but by then KDE Neon User edition should be "ready for the masses" - both are based on stable ubuntu LTS 16.04 OS and packaging/security updates system.

If you want latest and greatest now (impatient) for both OS and KDE then maybe try Arch-based Manjaro linux which should get you latest OS packages along with KDE stuff and you don't have to wait for Kubuntu/Neon to catch up, but note you will need to learn a little bit more for Arch packaging/setup etc. though they do have a very good wiki/forum for support.

HTH

Thanks for the good info.