While not a bad thing, or something that the customer should care, but it may influence the quality of translations since Transifex themselves don't have direct control on the translation process.
OneSky has in-house bilingual translators who are native speakers of the target language. This and the fact that they are in total control of the translation process ensures a high quality work.
Stringify focuses on developers. The powerful CLI enables you to automatically synchronize strings with the server and merge contributions from other users.
Stringify is easy to get started with and focuses on the features that you and your client need the most. Besides that, the web interface looks neat and clean.
The powerful CLI enables you to automatically synchronize strings with the server and merge contributions from other users. The developer is in control and decides when to synchronize the strings.