I MUCH prefer the Box size over the Twiggy - any day!
The reason is that it doesn't drag down at the ends like the Twiggy - many don't like how their stuff tends to migrate to each end of the Twiggy. I think the Box holds better and doesn't look massive, yet fits in a lot.
I don't know how the shoulder strap compares to the First, but the length of the straps that come with the Twiggy and City style is 24 inches long and the one on the Box is 26 1/2 inches long. I personally don't use my straps often buy carry my bags on my forearm mostly, and can still fit the handles on my shoulder if I want to. The Box is much larger than the Mini-Twiggy, and I think the style is really cute looking. Let me apologize to pp100 in advance, but in this photo you can see the City, Twiggy, and Box style pictured together. I love the Box!!!