one quick thing which comes to me is that Vertica is free only till 1TB with community edition .. but after that it has crazy licensing fees but that not the case with MonetDb .. you can freely download it an use it with any amount of data.
Vertica is more Entreprise level and have very detailed documentation but for Monetdb you have to make it work and try to figure out things by yourself and mostly by community.
Good thing is Monetdb is the only Opensource database which can challenge other paid Columnnar based databases.
Community can add more points here ..
Thanks