I'm going under the assumption you are looking at the MM sizes for both, not PM sizes (unless I'm wrong).
I got the Totally MM first, the Neverfull MM second. Not saying that you should get both (you can be happy with just one), but I like them for different reasons.
What I like about the Totally MM: Not a common bag at all, secure zip top, very comfortable (the bag hugs around my body, which I like), has lots of pockets for easy organization. Makes a good work bag.
This is my personal view on it, but I don't think the Totally MM is as big as some people make it out to be. It is a structured bag with a zipper and its interior lining cuts the amount of space in the bag. It's definitely more of a purse than a tote.
What I like about the Neverfull MM: Very easy to carry, fits a lot (more than the totally), easy to throw things inside and go. Makes a good travel and shopping bag.
Interestingly, I haven't had issues with the straps being uncomfortable with the Neverfull but I don't like to overstuff my bags. The only thing I really dislike about it is that it is very common and I get self-conscious when I see someone carry the same bag as I am (I know it's a mental thing but still). A true tote bag.
Hope this helps...