You might also contemplate the opportunity cost in holding the regular bond -- when interest rates go up, you are stuck earning the same old coupon (although it will be a 6% yield to maturity on your now-lower bond value) and you don't have the chance, as the bond fund does, of buying new investments yielding 6%. Not sure if this matters much compared to just holding the now-6%-yielding bond, but it was always mentioned in my finance textbooks!
I have heard this argument played out in vivid detail in my house (my father-in-law only buys 'real bonds'), and have come to the conclusion that some people just really worry about principal risk and there is no point in trying to change their minds. It isn't making one wrong or another right -- it is just a way of looking at the world and feeling risk.
And some of 2B's other bond fund quibbles have merit -- for instance, there are fees, although Vanguard's bond fund fees seem pretty de minimus.