The thing is that they are two completely different products. The only things they have in common is that they are both aimed at game-makers and they were made by TGC.
FPSC will make any
game you want - as long as it's an FPS. But they are of very, very good quality and relatively easy to create as much of the hard work is done by FPSC.
Dark Basic on the other hand will make any
program you want. Period!
It will also let you write utilities and other non-game programs as well as 2D and 3D games. This is because it's a programming language - not a game creator.
Unlike FPSC though, the quality of games you write with DB will depend on how good you get at programming with DB. It's far from guaranteed like with FPSC because
you are totally in control. So for a long, long time, all the programs you write in DB will probably be rubbish!
Learning is NOT easy. Think of it as being like learning another language like Russian or Greek - extremely hard at first, but it does get easier as you go along - if you put the effort in.
So, how easy do you want the learning process to be, and do you want the ability to write things other than FPS's?
Take a look at the tutorials for beginners on the DB Classic forum to give you an idea of using DB and download the trial versions of both products to have a play with them.
TDK_Man