If I were you I would get a back pack first to see how it goes, if you do mainly houses as I do you may find it easier and quicker doing them trad, I changed over to wfp earlier this year but found myself getting the ladders out more and more. Now I use mostly ladders, and just get the wfp out for the odd high window or for commercial jobs.
WFP definetly has its place and for certain work is a good tool, but it does tend to get over rated on forums like this with exagerated claims about how quick it is and how much money you can make from it. So my advice is get a back pack or make your own DIY system up as it is really very simple, don't get ripped off by the wfp suppliers who charge a fortune for what is a very simple piece of kit!