It has potential but I wouldn't recommend using DARK XL until it's finished, if I'm remembering correctly it's only in alpha at the moment. I have experienced many major issues with it, a lot that aren't immediately noticeable. Other than the fact that many people have reported that they weren't able to finish the game, there are also issues like there being no music in cutscenes or the voices dropping out in some of them. Some cutscenes are even missing, like the one that's supposed to play after you plant the charge on a reactor and escape. There are gameplay issues as well, like the enemies being able to see in the dark and the headlight having a very limited range, making fighting enemies in the dark frustrating.

Just play the DOS version. It might look and control worse, but at least you can finish the game and not have to deal with any annoying gameplay/graphical issues.

I agree, at present play the DOS version via DOSBox. But as for it controlling worse, there are ways to improve it. You can get a rudimentary mouselook by using GlovePIE to translate mouse forwards/backwards to the Look Up/Down keys. You can use DOSBox's own keymapper to move certain keys around, too, if you need to. And DOSBox can apply a filter to the graphics to smooth out the pixels if you don't like them (I stay with them myself, but I can see why some might want to smooth out the display).

There's a guide to setting it up as best as possible on the Steam forums. I posted a link to it on one of this game's forums here on GOG.com, too, so it's around here somewhere.