Thinking about this when I put two spaces into a sentence just now, it occurs to me that the author han't really addressed the size of a space, especially using proportional fonts. Are we talking n-spaces or m-spaces?

Typing into an email in Arial, I find that a space seems to be smaller than an n, and is certainly much smaller than an m; in fact, two spaces seem to take up less line than an m does - and that's talking lower case rather than upper.

So in a proportional font, using two spaces is simply making sure that you get something approaching a single m-space where the space is most important. In effect,hitting the spacebar twice is simply the way to insert an m-space rather than an n-space. So if you wanted to reconcile the two views, there's a nice compromise: one space is correct ; it should be an m-space; hit spacebar twice to insert it :-)