In my limited experience with GPS units, I've found that portable ones tend to have lower contrast displays simply because they have a weaker power supply.
Built in ones tend to be brighter and easier to read, but I would presume use more power than two or four or six AA dry cells can provide.
But, to help you find a good one, I prefer colour displays - seem to be easier to read / interpret in difficult conditions.
regards,
O8
PS Your sunglasses will have a bearing on this problem - the more expensive, the better generally! Cheap ones tend to block out lots of light, but better ones block glare and UV without blocking too much "reading light". O8