Background: I've been commuting in the DC area for 16 years, riding a German Kettler Silverstar with a step-through frame, Nexus 7 speed, and front and rear roller brakes. I figure I've put around 50,000 all-weather miles on it. That's included five years pulling my daughter in a trailer and five years on a Trail-A-Bike. The bike is similar to the Twin City except it has hub dynamo lights and front and rear rollerbrakes.

My understanding is that the five speed Nexus has a smaller range of gear ratios than does the seven. There's no way I'd want less than the seven, given some of the hills here. That is even more true when pulling a child. Get the Nexus 7; don't even think twice about it. I've never ridden a CaBi bike, so I can't comment upon their gearing.

Before I got my Kettler, I rode a hybrid with rim brakes. I went through a front rim every winter, due to all the sand and salt they dump on the roads. Replacing brake pads was a hassle, and braking in the wet and snow was not satisfactory. The Kettler was a big relief. I'd stick with the rollerbrake.

Most of the Dutch and German commuter bikes have integral rear racks and child seats are compatible with them.

Bike shops raise stems all the time; it's an easy fix.

Panniers are also a good idea. Some stuff does not like to be lashed to a rack.

The only other upgrade I would recommend is lights. A good dynamo system will put you in Soma territory. I think the convenience makes them a bargain. (I started riding because it was more convenient than driving, so I will always make "easy" choices.) However, a rear blinky and battery headlight, inconvenient and fiddly as they may be, are a cheap way to get a useful bike.

Paul