I've used PureBASIC for almost a year by now, and I can tell you DarkBASIC (even Classic) totally owns the PureBASIC 3D way of programming.
3D:
PB uses a kinda-interpreted version of OGRE (I mean it doesn't uses it "raw", but it has its own way to handle it - kinda like DB with DirectX). The commands were wannabe-easy for the programmer but they aren't yet... still need work. It's a bit more advanced than DB in the sense you can use OpenGL, DirectX, Irrlicht 3D Engine and lots of wrappers for them, and do stuff that DB Classic can't (but can't do stuff that DBPro can
) but isn't the same - get Blitz3D then... but DB pwns it too
.
DB Classic & Professional are DirectX wrappers and their programming is WAY easier than in PureBASIC, doesn't use the OGRE 3D engine and they're the best option for 3D programming. DB Pro has the functions that PB can do and can't do too, such as it can handle the program in a window with less bugs and stuff than DB Classic AND it allows to use buttons, frames, text fields, basically windows stuff inside the 3D screen - something I haven't seen PB able to do yet. Also I've seen some tweaking made in DBPro apps to make games using OpenGL instead of DirectX... which shows how expandable is DBPro - even if it's tweaking the engine a bit ^^.
2D:
This time PB owns, its way of programming 2D games is really easy (of course by using PB-made DirectX wrappers). You can also access the DirectX 7 directly and use alpha transparency, blending effects, etc. But some things are harder to do (uselessly, 'cause it could have been done in another way that somehow the PB makers didn't added functionality / commands to do so) such as getting the FPS (which I consider hard) and other simple stuff.
DB Classic has its downs on this: It has some nice (but basic) 2D functions but aren't as good as to make, let's say, a Final Fantasy 5. It's POSSIBLE, but really hard with the limited 2D functions it has. DB Professional improved on this, but both DB are focused more in 3D than in 2D game creation.
Conclusion: If you want to make the latest 2D hit (a Diablo II clone... or something similar) get PB... BUT if you want to make awesome 3D games using shaders, bump & reflection mapping, ability to use X, 3DS, MDL/MD2 models, BSP maps, simple & easy collision (using Nuclear Glory - cheap) and using a really easy programming language get DarkBASIC... it's the best thing you can do
.
