No it does not come down to personal preference, except that the writer's proper preference to produce more readable writing. In fact one thing I particularly dislike about HTML is that it (usually) automatically collapses two spaces to one. And conventions are only good when they are better than the alternatives - two spaces helps set off a sentence, just as capitalization does, and makes text more readable. Web "usability" is also strongly against long blocks of text, which tends to suggest (to me at least) that non-readers (or even anti-readers, witness the popularity of videoblogging and podcasts) have too much influence over web conventions.