The Pixel Qi is an advanced version of the display used in the XO/OLPC (One Laptop Per Child) computer (which I have). In sunlight, the color fades to black and white, but the horizontal resolution triples (RGB triplets become visible as individual pixels). The only downside is the shiny screen making the sky and clouds visible unless you adjust the viewing angle to minimize that. I prefer the matte surface of a Kindle eink reader outside.
From watching the mirasol demo videos online, the mirasol displays appear to get washed-out color when viewed off-axis. The current mirasol devices are being discontinued and sold at clearance prices ($87USD ?), mostly due to bad software. Changing the viewing angle to minimize reflected sky and clouds could also detract from the colors due to off-axis viewing.
The new color eink devices suffer from poor contrast and somewhat washed-out colors because they are made of a black and white eink display with an overlayed color filter. The color images are formed of a pattern of dots similar to viewing a comic strip in a newspaper. Because many pixels are NOT white (from the color filter overlay), the overall contrast suffers.
Because all three of these outdoor-viewable color display technologies suffer from different technical difficulties, which one you find least objectionable for your viewing habits is purely a subjective personal preference. You really need to try each of them indoors and outdoors in the sunlight, before making a firm purchasing decision.