If you're a user then it doesn't matter what distro you use, they're all fine.
Not at all true. Lots of people prefer Mint, for instance. I'm one, and one reason is for me the Ubuntu gui is just awful. Or what if you need really high security?
One of the main things that scares people off of Linux is the hundreds of distributions. Telling them it doesn't matter which one they pick isn't going to help with this.
I agree that the variety of distributions can be a challenge for new users. One of the beauties of FOSS is that it's all open and consequently easy for moderately technical end-users to 'remix' and create new. One of the challenges is that many of the distributions have minimal differences between them, in my opinion it's better to get to know one of the underlying system families and stay with that family just adding/changing components as you need them. Changing between distributions because of the GUI feels like overkill to me when you can easily install a different desktop environment or window manager.
Lots of existing users prefer Mint because they transitioned when a roughly equivalent looking GUI existed on the proprietary OS they were coming from. Everyone prefers familiarity - and that's fair enough. Ubuntu uses a GUI that's aimed to target new users [0] - the Ubuntu GUI is far easier to use for contemporary general computer users [1] - if you're an advanced user then the freedom of choice of Linux is still there in the X thousand packages in the archive!
To install a different desktop to the default is pretty simple: install the desktop package and edit the session file, or do it the old-fashioned way and edit a .xinitrc. Any advanced user should be able to do this, in fact it's part of the joy of investigating the options! For example, I use i3.
If you care about high-security then by definition you have to use one of the distributions which has a full-time security team. Looking at CVE dates and when updates are published would be revealing.
[0] Source, I used to work for Canonical.
[1] You may find this surprising (I did) but the vast majority of end-users really don't do much with 'right click' and find virtual desktops utterly and totally confusing, they often don't even know about alt-tab.
Not at all true. Lots of people prefer Mint, for instance. I'm one, and one reason is for me the Ubuntu gui is just awful. Or what if you need really high security?
One of the main things that scares people off of Linux is the hundreds of distributions. Telling them it doesn't matter which one they pick isn't going to help with this.