Regarding screen sizes, in this case big is definitely beautiful , less redrawing and more detail on screen. Black and white versus colour I would say no contest, I have regretted getting b&w although the set I have is a great improvement on my previous gps which by the way I kept as a back up and still use regularly
At the end of the day you pays yer money and makes your choice. All will do the job its just that some are nicer and easier to use than others
Try http://www.redcar-fish.co.uk maybe not the cheapest but they are very helpful with good after sales support
Martin