The short answer is that it depends on where you are starting from ...
My background is that I started programming for PCs using Blitz Basic (the 2D version) a few years ago. This was relatively easy to learn as I'd programmed in BBC Basic quite a few years previously (what a machine the Acorn Electron was ...).
Once I'd mastered 2D games, I wanted to go further and move onto 3D. So I bought DarkBasic Classic, but didn't find its performance satisfactory - particularly, when it came to local and global variables and recursive functions (which I use for pathfinding).
I've recently bought DB Pro, which is far better IMHO and overcomes these drawbacks. I'm also amazed at the quality and range of free plugins available for DB Pro.
I'm not sure where Blitz 3D is going - my impression was that this software was stagnating and not keeping up to date with DirectX development (not an issue with DBPro).
In terms of how easy I've found it to pick up DBPro, then I've used the approach to work throught the tutorials and learn from others - and by adapting the tutorials to make games of my own, I'm making good progress and understanding how to overcome the issues that I face. I'd expect to have done something similar if I'd bought Blitz 3D.
I've found both the manual for Blitz Basic and DB Pro didn't really help that much once I started to write my own games (and once I had worked out what a type was). Hence my focus on tutorials and the forums.
So to summarise, I'd recommend DBPro (what a surprise
).