Main differences? From what I've gathered:
-GDN prices are higher for banner clicks
-GDN has more complicancy issues - more difficult to get aggressive banners approved (eg. shocking-news type)
-Can't blacklist carriers, devices on GDN. Although can blacklist placements.
How big is the bot/clickfraud problem on GDN? As bad as Decisive, Airpush etc.? We can blacklist placements on GDN but not carriers, devices..
Thoughts? Please correct me if I'm wrong 
Scratch the last point - we can actually target carriers/devices on GDN https://support.google.com/adwords/answer/1722028?hl=en
Still though - the guys that are running GDN traffic for mobile and track your traffic - is bot traffic and bad placements even a problem? How much are you blacklisting placements? (compared to say on Airpush, Inmobi) Thanks! 