I put both, though in retrospect I would prefer 40mm, I've got 1 or 2 40mm figures and they actually do work great as larger folk - some Norse inspired 40mm would be greatly thematic against the various celtic 28-30mm figures I've got.
In some comic-esque or Manga-esque RPGs it does sometimes come into play having someone literally larger than life.
In short, 40mm I can actually use in gaming, while 54mm is essentially just a display piece - and my painting standard isn't high enough to justify (though if I take a shine to one, I might pay my artist mate to paint it up - like I did with the 54mm? Deneghra from Privateer).