I am middle age man seriously considering to build a 3 stringed CBG. I have a couple of questions about tuning.

When I was young I used to play a normal acoustic guitar. Since then my voice has been changing so much that almost every single song I have to take down 2 full steps, like from E down to C. Now if I build a new guitar I have the thought of making it easy to play most songs 2 full steps lower. That gives the first question: taking this into consideration, what tuning would you recommend me that would allow me to play chords for most songs close to the nut.

My second question: why is an open power chord tuning preferred over an open major chord (like G D G instead of G B D)? An open major chord would make both major and minor easy as minor chords would be formed in the same way as D7 on a normal guitar. I am sure there is a real reason for going open power chords instead, that me as a newbie (who has never touched a CBG) can't see.