I'm going to be honest here. The Darkglass is a great pedal, but if you've never used a compressor before you should get the Keeley. You can't mess it up and it's very clear to understand what it's doing. I used the Darkglass for a while and ended up back with the Keeley because I never stopped adjusting the Darkglass. It has three different compressors built in, each with 4 different 'voices' (or 5 in some cases), plus the time and blend knob...and that's before you get into the software where you can adjust the high pass filter, attack, release, and other stuff...although you don't HAVE to mess with the software.
I really really like the Darkglass but one day I realized something....I spent hours upon hours trying to make it sound like my Keeley Bassist. So I just went back to the Keeley Bassist.
But that being said, the Darkglass does a ton of stuff that the Keeley doesn't do. But that may not be a good thing for a first time user.