I thought the ZW came out first.
Well... I was interested in the MC-402 but a lot of Harmony central reviews slammed the OD sound as too fuzzy so i didn't bother. I already had the ZW and microamp but liked the look of it but ultimately the reviews put me off. I figured the GT-OD was the single pedal overdrive of the MC-402 like the MC-401 is the clean boost side of the 402 but I could be wrong.
The ZW is a very cool pedal it does sound 'thinner' than the microamp though. I swap them out every so often just for kicks although I prefer the microamp it works with my setup so I just have it on 10 and leave it.
The ZW sounds way better than a DS-1 at bedroom volumes for reference although that's just my opinon.
Hopefully the GT-OD is just a green ZW for the folk who don't want to be associated with him in which case it's a great pedal.
If you can't try them both I'd say get the ZW as it's cheaper and will probably be easier to find and try out. I don't think you'd regret buying it unless you went way more expensive and got an Xotic BB preamp, Xotic AC + or Fulltone Fulldrive II if your a tone hound.
there is the new Satch Pedal which looks about the same price and is hopefully as good as the ZW.
What amp are you using and what kind of settings / type of sound are you looking for though? The ZW sounds great with a
tube amp set on crunch to give it that rock sound ala well duh Zakk Wylde. For turning a clean amp into a distorted one I'd say get an Xotic. They are a lot more expensive but they sound great.
P