Get the Damier! Since the speedy is handheld, I find that it's best that the handles are dark so they don't show as much dirt. And get the 35 size--I have one in mono (I have a NF in Damier) and I love the size!
Wow! This is a tough one! I like both also and I have the 35 in Mono and really enjoy carrying it and I have the Damier Ebene 35 on my wishlist ! So having said that, since you have one piece in Mono, I suggest getting the Ebene. But then again, you can't have too much Mono!
I vote the Damier, so classy & elegant with a pop of color inside. Get what you ultimately like and will carry. Don't get caught up in having a bag/SLG from the different lines unless you know you will carry & use those items. Get what you like.
if tivoli is an option, i say tivoli but between the two speedys, i say go for the damier since you already have something in mono. gl deciding! keep us posted.