My vote is for a shared stash. There really is no right or wrong answer in this argument between shared stash and mail system. When you boil them down, they are really very similar. It comes down to feel, and for me... a shared stash just feels better. More diablo'esque (that's for you Risingred).
You can purchase upgrades for your stash to increase its size. Last we heard, a fully upgraded stash will take up 2/3 of the screen. That's huge! It seems safe to assume that you will also be able to purchase upgrades for the shared stash as well.