The Garmin and Avidyne panel screens are really superb. I think the Garmin is marginally better but I bet some one will be along to tell me both companies use the same panel! I wonder. Interestingly in the marine wrold Raymarine have produced some very good sun light readable products for quite a while that are very well priced. Again they are not portable but it does demonstrate the basic panel can be built into to this type of product without a significant increase in the unit cost.
In all fairness these are powered by the aircraft systems. Most of the mobile units produced are designed to run off battery power, and the manufacturers face the issue where increased brightness results in either very short battery life, or requirement for a much larger battery.
A much larger battery = more expensive and heavier, both of which generally reduce the appeal of the units.
For most users of these devices, sunlight readability isn't a major concern....it's just us pilots that see an issue.
Some of the car nav units, have very good sunlight readable screens (high end Navigons for example) because they are designed to be used where sunlight gets at them (car windscreen), but obviously these are too small for showing approach plates on.
dp