There are no cheap tablet devices which are truly sunlight readable.
A sunlight readable LCD needs an extra layer on the front, and for curious marketing reasons these come only on ~ £2000 products, despite costing only about £200 extra on those products.
Here are some comparison shots I took the other day, on a flight, of my old Motion LS800 tablet (cost £1800 in 2005) with a sunlight readable screen, next to a new Viliv X70EX (cost £600 now) which is shown with an without a stick-on matt screen protector.
Judging from pics I have seen, the X70EX has similar sunlight readability to the Ipad, again with and without the stick-on matt layer.
You can see the normal LCD is OK in a shaded situation but virtually useless in direct sunlight. It boils down to whether you are able to re-orient the thing.
This is an example of a current sunlight readable tablet product (£2000). I have seen this for real and it is stunning. At the price, it ought to be
E-book readers such as the Kindle DX are perfectly sunlight readable but nobody has yet found a way to run a GPS moving map on them. They make fine document readers for approach plates, manuals, etc.