How to set Mail.app to ask what is the sending address on new mail
The easiest way (at least the one I know of) is simply selecting the mailbox on the left you want to send from, and then click the new mail icon.
It will put as send-address the mailbox you selected on the left.

You can always use the dropdown afterwards if you want it to be another address, but that's the easiest way.
To make it more convenient, set the boxes as top 2 and you can easily select one of both.