Thank you everyone for your replies. I booked with Hertz in the end. It worked out that very few companies would allow me to pick the car up in Boston and drop off in Chicago. Have booked a Toyota Camry, which is part of Hertz "green" range.
Did you book via the UK or US site?
In my experience, normally if you book via non-US site, the CDW is included in the price, if you do via the US site, it is not.
Be aware of this, as I have had multiple occasions where at the desk they ask if I want the CDW, I tell them I already have it, they say no I don't, I show them my paperwork from the booking, they tap their keyboard a lot and then everything is ok again.