In reply to steveb2006:
They're comparable in many ways and most people seem to enjoy them "in addition to" rather that choosing one as an alternative to another.
Whilst they both enjoy the achievement of going from 'coast to coast', they also share some of the draw backs this entails: the route isn't free to find the best quality cycling as at some point you have to get to the coast! So there are some sections that clearly are included through necessity rather than merit.
They also share the perversity found on all designated cycle routes in the UK: the designers will go anywhere to avoid 'traffic', so you have the inclusion of gratuitous hills, stretches of 'cycle paths' of questionable quality, apparently random diversions and mixed surfaces including mud/ rubble / flytipping / cobbles to negotiate. I think in the planners mind they visualise everyone on some sort of go anywhere, do anything type of hybrid which no one doing these routes actually rides.
Ultimately these routes are only where 'someone' says a route exists, often with a large input from local authorities along the route. And there are others to consider including Hadrian's Wall which people rave about, Walney to Wear which looks good and a DIY creation from Ravensglass to Whitby which with a bit of care can include a phenomenal amount of climbing, albeit on some busier roads.