I'm not sure if you're aware but you have a bigger choice than a windows tablet. Not that that is a bad choice, but you do have others.
You can use a car nav unit or windows smart phone or pda with many systems including PocketFMS and SkyDemon. You used to be able to use Memory Map with some of them but it required some hacking, and I believe they Memory Map have taken away the ability to do that now.
You'll probably find that a car sat nav is cheaper than your small tablet, and a good own will probably have a better sunlight readable screen. Obviously though the screen will be smaller.
dp