Gengo is catering to the professional translator. Each job is done by one person, with the possibility of review (costs extra). Unbabel is crowd translation, this means that multiple people work on the translation. Regarding QA, only the top unbabelers have access to paid tasks and to get there they need to have really good feedback from the community.

Depending on your content, Unbabel quality is quite good. Our hope is that it will be as good as Gengo.