I like the size down from that. It's 2 paper grocery bags in size, and it's deep enough to hold stuff in. Those flat rack are much more bungee dependent.
I've been riding with the size smaller for 2 year now and it's been great, but it does change the handling. Between the weight and the flex, even when it's empty it's enough to notice and make riding no hands pretty tough.
I do recommend it for a townie because it's nice to be able to carry so much on a whim, and you're not usually on a townie for the speed and handling.
Edit: I'll add that you should really have a bolt on axle for one of those. It's not good to run it with quick release and it won't mount easily anywhere else.