Take your pick:
Wheelbase broader than the workpiece makes the Kell guide master of the blade, so it dictates square rather than accepting the blade's version of square. - Admittedly the Veritas is better than most in this regard.
A wedge will hold a blade more securely than a bar with a screw at either end - if you don't get the screw tension absolutely identical the blade will clamped more firmly on one side than the other.
With the Kell, if you make your rails a bit longer than the stone you can use the whole surface rather than having to subtract the distance between the roller and the edge.
Cheaper? Kell = £44.95, Veritas = £50.95
Easier to use? I'm not so sure, the Kell requres a little lateral thought but in use you just set the projection, squeeze the wedge home and hone. The Veritas is more prescriptive but it has more knobs and switches than a Norden bomb sight.
Will the Veritas do short bladed chisels, mortice chisels, skew chisels, fishtail chisels, tapered irons, plough plane irons, shoulder plane irons, scraper blades...?
Maybe I'm being too harsh, it is definitely the other best guide available and I do recommend the Veritas to anyone who doesn't get on with the Kell (and we don't even carry the Veritas). Like the rest of their product range it is certainly a well thought out and engineered bit of kit.
Personally though, I just love the simplicity of Richard's design - only two moving parts (the wheels) and yet with the application of a bit of nowse it will facilitate the accurate honing of just about anything in the 'shop. Somehow it's a more 'British' solution - 'You can keep your Norden bomb sight thank you, with a V shaped stick, two lights and a spinning bomb we can flood the whole valley!'