Well, I guess I'm biased because I keep breeding Rosemaries!
cmaec makes some very good points. You need to ask yourself those questions.
When my very first Rosemary arrived I was shocked, it looked so tiny, but it is like a tardis, bigger inside than you realize. I do have a thread somewhere that shows content comparisons, let me see if I can find it... aah, here it is:
I personally find Rosemary to be extremely versatile since it's comfortable over the arm or the shoulder, but of course it wouldn't be practical if it didn't fit all your stuff! I personally don't find Roxy very comfortable over the shoulder, it gives me 'chicken arm', but maybe that's because it hasn't softened up very much.
- Are you a big bag person or a small bag person?
- Do you like a loose or tighter fit over the shoulder?
- How important are usable front pockets and what would you put in them?
- Definitely need to ask yourself what you need to fit in your bag, that should determine which size will suit you best.