I think go with WFP, but ensure you keep your hand in on traditional methods, as you WILL get customers who want their windows done trad (especially thre older generation in bungalows). WFP allows you to add aditional services to your business that using a ladder, blade and mop do not (facias, soffits, exterior of guttering, windows above conservatories, Velux windows etc).
WFP and blade are both just tools of the trade and both have their place. Personally, I use WFP for all but 1 of my customers, and they are all more than happy with the results. There is no reason why WFP would only be good for anything above 1st floor, just as their is no reason why trad would only be good for ground + 1st floor.
Keep your trad gear in your bucket in the van, and a step ladder and a 2 / 3 part ladder on a roof rack, with your WFP gear and you will never have to turn down a job.