which Marc by Marc Jacobs should I buy?

Well they are very different, especially in size. Are you wanting a smaller bag or a larger bag? There is going to be a pochette with the dove on it, which would be my choice for a smaller bag because I really like the dove motif. If you are wanting an everyday bag to carry all your stuff, then definitely get the Sasha.
 
I vote for the dreamy pouchette! It's so pretty and not too large, I think it would be perfect for shopping so you can wear it crossbody if you need more hands to hold your purchases, and you can also wear it for evening activities....Love that! Good luck....