Calibre Companion is developed by a calibre developer, who is active here (help and bugfixes) and personally wrote the smart device driver in calibre (which the app uses to communicate). He also knows a LOT about how to work directly with calibre.
Several other apps exist which utilize the smart device driver, but it was written and is developed primarily for Calibre Companion's needs.
Whichever app does what you need, get it. Keep in mind, chaley is a known quantity and someone you can easily contact, responsive about issues, requests, and bugs...
And people tend to appreciate that a lot here. In case you didn't notice, I am highly biased towards Calibre Companion.
