I don't have a twiggy, I have 3 Citys.
I think the city size is very confortable, not to small so that you can bring with you some stuff you need during the day, not to big to look wrong with an outfit for a night out or for shopping either. It looks good with most of the ways you will probably dress for your lifestyle, slightly more formal, slightly more sporty.
Twiggy has about the same use, I would say the difference is that city is basicly deeper, so it can hold a little bit more.
For eg somebody say a few days ago she managed to fit a small umbrella in the city whereas it didn't fit in the twiggy.
If you don't need to bring a lot of stuff with you, I would say you may use both in the same way, so it just comes to a matter of taste.
Aesthetically speaking I prefer the city, it's just a matter of proportions and because I love to see the softness of the leather ,and I see this better in the city, but I honestly think the twiggy is a very very nice bag as well.