Note that I have 2 main requirement : 1. Open source. 2. GPG integration

Mutt on laptop, k9-mail on android.

mutt ++ : just do what you ask him to do. lighter than any other solution I've tried. you don't need your mouse to use it.

mutt -- : Not always packaged with a conf. a bit confusing at first

k9-mail ++: GPG integration (using another app). incredible tunning level. well integrated with android

K9-mail --: few UX issues