I would recomend you go dualboot as soon you have decided which linux it should be.
As you want to learn, VM's are great, you can do things without breaking your system.
On the other hand, if you have the system installed for real, you have situations to tinker, you wouldnt have with a VM.
AFAIK:
Red Hat is widely spread in business use, to get used to its mechanics, you can also use Fedora, CentOS (Server'ish oriented) or Scientific Linux.
IMHO:
Ubuntu is the best choice for a first experience with linux, it makes the switch from Windows (or to adapt the 'basic-usage-thoughts').
However, rumors say they collect data, and present ads on their dash... I dont know!
Mint would be the 2nd choice for switchers.
Debian is also a great choice to learn linux.
Its one of the oldest still around 'as is/were', and also the base for Ubuntu and Mint and also often used in business.
Arch, Gentoo & Slackware are for intermediate linux users, or for people learning incredible fast.
I consider myself as a fast learner, but the freedom (and some 'must's') i had was too much to handle at first contact.
There are Sabayon, Kali or Mandriva around, which are great distros as well, but they are far less spread.
Conclusion:
For newcomers i would strongly recomend to go for either a:
- Red Hat - based system (CentOS, Fedora, Scientific Linux)
- Debian - based system ([LXK]ubuntu, Mint)
That is because they, and their package manager, is used in the many places and their package managers as well.
My favor is Fedora. It release cycle is ~6 months.
New technologies are also pushed, one might not like it all the times though (systemd)
And a warning ahead, there are also diffrent DE's (Desktop Environment) and WM's (Window Managers) around.
Just because a distro shows that one, doesnt mean it has no other in its repositries.
But not every distro has every DE or WM available.
In the end, to learn the most, there is only one way to go:
LFS Project Homepage
Hope this helps
(and that is just the linux side, idk about Unix/BSD)