I will say one thing for books - pdf's ar every handy I 'spose, but a physical product can be sold or traded when one no longer has any use for it on Ebay etc. I bought a lot of my old RQ stuff second hand, and a pdf can not be traded the same way. Therefore if for any reason the company publishing a supplement ceases to exist, that information is forever withdrawn from circulation, pending selling on of publication rights - whereas with books one can theoretically order them from alocal library in extremis. :)
cj x