> the Guinness head means you're getting less than a pint of actual beer

I hate to be pedantic but pint being volumetric, you're still getting a pint, independent of density. Also - a nitrogen head doesn't dissipate, so you never get a gap.

I'm now curious though whether a nitrogen head is less dense than a CO2 head...

It feels much denser, and I think it does dissipate... but slowly.