Manchester to Blackpool is a more scenic route, but these rides will attract the once a year charidee riders who don't know cycling etiquette, last time I did the M2B I spent most of the time following the white line in the middle of the road to avoid the unpredictables riding.
I always think they're good – raise money for charity and get people out on bikes who may not otherwise ride, and some of them may become more regular cyclists.
Last time I did Manchester 100 I was impressed with the two young teenagers who were riding the cheapest heaviest mountain bikes all in aid of Christies in Manchester.