I own both Simpson and Day, and Morcombe. I have had S&D for at least 5 years and it has been my constant travel companion and has served me very well, though at times I have wished for a second guide. S&D is nice, the illustrations are well done, but admittedly the plates are confusing with the orientation and placement of the birds, and there are flights views where I feel they're not needed and no flight images where I would really benefit from them. Towards the end of last year I invested in Morcombe, the additional text is a valuable resource and I've found myself comparing images between the two books. Morcombe's images can be a bit static, but I sometimes find that the simplicity of them makes for comparing ID features of similar birds easier at times, although it does depend on the bird and images. The layout of the images is much nicer, makes great use of space, plenty of flight images which is fantastic and very useful and the placement of the images next to the relevant text makes figuring out what bird you're looking at much simpler. It's a bit bulky for me for when I'm walking, I always carry S&D when I'm walking for quick ref. and I either carry morcombe in the car or just compare when I get home. When I was researching for a second field guide, I was preffering Pizzey and Knight from what I was seeing online, but then I got to the store, and flicked through both and Morcombe just made much more sense to me.
I've had the opportunity to use Slater on many Uni camps, but to be honest, the perfect layout of the illustrations always put me off for some reason, although I guess it is a good comparison, it just seemed to lifeless to me and I could never get the feel for it.
For most things, I find that one guide in the field is sufficient, especially now that I know my birds better, but I know for things that I don't know, like shorebirds etc, having an additional guide to compare to is really valuable.