I suggested Kubuntu because it's based on Ubuntu which seems to just work for the majority of people. Personally I'd go for Gentoo steep learning curve though.
Choosing a linux distribution or just even suggesting one is a difficult thing to do. We all have our favorite disro. For the general user who wants to learn I've found Ubuntu to be the best choice as it normally works straight out of the box.
Yes, I said 'buntu is a good place to start.
"Also a good thing to remember is if you don't like the WM you've chosen you can just remove it and try another."
That's what I've been saying, there is no need to remove the old one.
Mint is already installed, which IS Ubuntu, just customized. Installing the package kde-desktop or for sure kubuntu-desktop will pull in everything, logging out, selecting kde, logging back in is -basically- the same thing as installing Kubuntu separately (other than more software). If Op doesn't like it, log back out, select gnome, log back in, back where he started. Similarly, if OP had installed the package xubuntu-desktop, logged out, selected XFCE, logged back in, could have looked at it that way, except it wouldn't have been customized the "Mint way".
Gentoo is far too advanced for a general beginner, unless they want to do a fair amount of reading (to do it the right way anyway). There are no/negligible noticeable differences in speed, haven't tried it for a couple years, with all the time compiling software, I don't see using it full time myself. But yes, you do learn a lot about linux.
If I were to run what I wanted, it would be Arch, but I'm on a limited internet connection, it's easier for me to keep up multiple systems with something simpler. And since most people I know don't have broadband because it's not available, and I have a couple neighbors running it, it's easier to copy over the packages to save from downloading again, and the package manager is decent/handy. They don't have to touch a command line, which they wouldn't want to. 'buntu allows easy remastering so I can simply install my existing system on anything I want.
Choosing a distro is subjective. Day to day usage, just using apps, they are all very much similar. The differences are in some of the tools, installing software, etc.