JavaScript asynchronous loader 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 

Because they are so recent modules and you can't know how well they will be supported later in the future, I would  base my decision on which JS library fits your needs. Probably I would trust more on LABjs, that is much more tested and used (twitter, vimeo, etc are using it), plus it's a library focused in JavaScript loading, nothing else.

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.

If you are interested in this topic, I would recommend you a must read blog post from the author of LabJS to get a inside view of the technology under these JavaScript loaders (LABjs, HeadJS, ControlJS from Steve Souders, etc )