I've used XCsoar on a Nook Simple Touch for a couple of years and love it. I use it with a GPS USB puck and a Y-cable for external power. There have been a few glitches/freezeups, but if I start with a fully charged Nook there is rarely a problem. (Be sure to return its USB mode to "peripheral" and then fully power-off after you are done flying.)

I have yet to see ANY display, at any price, that is more sunlight-readable than the e-ink display of the B&W e-readers. The hardware is a bit flimsy but replacements are still easy to find and cheap. I added an extension to the snap-on-back that I use to mount the Nook with, to protect the USB plug from being bumped by my knee.

The Kobo seems like another option, but for me the simplicity of using the USB port of the Nook as-is is a plus, as is the larger display (and it fit in my cockpit, just barely).