In the future I would like to upgrade a fully built differential. Should I go with 4.09, 3.90 3.69

 

As far as I know, you're limited in FD options due to what's available for your front differential gearing.

Your front gearing must match your rear gearing. The 07-09 V36 awd has 3.69 front and rear gearing you can use.

Manual G37 also came with 3.69, and to make the diff assembly work, you can swap the input flange.