Since you already have a color tablet, I would firstly look for an e-ink device. That way you will have something to read on when you are outdoors (assuming you like to read outdoors), even in bright summer light. It will complement what you already have.
The next thing you need to ask yourself is "do I buy books on impulse"? If not you'll be fine with any device, if you do buy on impusle then a Kindle, Nook or Sony have the advantage of being tied into an eco-system that lets you buy books over the internet. Can your phone tether with Wifi-hotspot? Then you can pass on the added cost of 3G for your device.
|