Which module (not library) works best in Drupal 7 (they support corresponding libraries), labjs or headjs? Which is one better written, tested, and with less bugs?
Both LABjs and HeadJS are recent modules, actively developed by professional Drupal developers, both are offering versions for Drupal6 and 7 and both have less than 100 installations (see statistics).
Also both are aware of each other modules and collaborate together, see
HeadJs library in the other hand also is offering other features: detecting CSS3 features, detecting screen resolution, etc something that can be good or not, depending what you are looking for. And also HeadJS is better integrated in the Drupal ecosystem: integration with Boost, CDN, Advagg modules.