which version of ROS to install?

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Disclosure: I develop on the ROS2 navigation stack. I'm a little biased.

This may indeed be the case.

For people just starting out, the fact that there is almost zero documentation and tutorials is a major pain point with ROS 2. Don't underestimate the difficulty newcomers will have trying to understand how things "fit together", how packages are supposed to be created, how workspaces work (Colcon? Catkin? catkin_tools? catkin_make?), which (build) tools to use, etc, etc.

Even something as essential as creating a new, custom message is already quite complex and somewhat underdocumented (the tutorial even acknowledges that).

And that's not even mentioning Ament ..

That's totally fair, however even with the abundance of documentation, ROS1 to many people is also a "trial by fire" experience getting up and running. But for all the reasons gvdhoorn mentioned, ROS1 Melodic isn't by any means deprecated. If you have a little grit to power through issues and already have some C++ experience, I don't think it's insurmountable.

ROS1 to many people is also a "trial by fire" experience getting up and running.

Sure, but at least there are at about 20+ books about it, MOOCs, summer schools, video tutorials, a large support forum + many more smaller ones and 10+ years of experience.

ROS 2 will have all of that, but right now I would not state -- without some nuance -- to newcomers to "learn ROS 2 if you're just starting out".

ok, here's an example. I start with ROS2 (Dashing). Get it all running on Unbuntu 18.04, deploy it to my raspberry pi powered robot, then ROS2 goes to the next release- maybe fixes some bugs, improves some things. I assume my ROS2 (Dashing) work can still be used in the next release? By the way, what do I call this- a "package" developed in ROS2 Dashing? Sorry for this basic question. I could not find a tutorial or process-flow that gives a simple general overview of the process, any clarity would be appreciated.

@jweaver and this is why so far it has been recommended to start with ROS tutorials. The tutorials give a good introduction into ROS and I would definitely recommend going through them at least once to grasp the basic concepts. Even with all the documentation and tutorials ROS has a steep learning curve and diving directly into ROS2 will leave you will some confusion and a lot of questions.

ok, I will do that, but if I have Unbuntu 19.04 installed, do I have to uninstall it and re-install 18.04? I mean what is the standard practice? Do people normally have different versions installed ?
Sorry- in this forum I did not see any "reply" option, only "add comment"

@jweaver this is not a forum, it is a Q&A-site, like stackoverflow. Thus no reply :-)

there has been the decision to support ROS distributions mainly for the specific Ubuntu LTS to lower the maintenance burden. Thus, 19.04 does not have precompiled binaries. You could look into installing from source, but I'd discourage that. better use 18.04.