Obviously if you encrypt client side, and can securely share a strong key, it does not matter where you store; MW and Dropbox would be equally secure in this re

Obviously if you encrypt client side, and can securely share a strong key, it does not matter where you store; MW and Dropbox would be equally secure in this respect. With MW you share a weak key (vs a strong key in Dropbox case). Dropbox also provides access to all data. On the other hand, you have to hope there is no vulnerability in MW. With Dropbox, you have much more control over crypto.