The Twiggy was my least favourite style until I finally took out all the tissue paper from one at Barneys and tried it on. Now I am getting ready to purchase my fourth Twiggy! It and the First are my favourite styles at this point.
I think the Twiggy is particularly flattering on short women; I am 4'11" (:s) and the City appears a bit big and square on me. The Twiggy is a more manageable size and holds about the same amount of stuff. Like
UliUli, I think it's easier to find things as well without the extra depth. Also, the inside zip pocket on the Twiggy is VERY roomy and holds quite a bit, whereas the City's is somewhat small and useless for me.
I should point out that my Twiggies have a relatively small haul stuffed inside of them. I think that if you are going to be cramming your bag full, the City is a better option - when the Twiggy gets stuffed, it does have an unappealing sausage effect. I prefer the Twiggy to the City for shoulder strap use as well - on a small person like me, it's very easy to wear messenger style if I want my hands free and it looks cute just slung over one shoulder with the strap too.
I say try a Twiggy!