Though I cannot find anywhere that explicitly states it, all officially released material leads me to think that potions consume a healing surge to take effect, while elixirs instead count as one of your item daily powers. Elixirs tend to be more powerful, given that you have many healing surges, but only 1 item daily power per milestone per tier (or would that be per tier per milestone?).
It may be mentioned elsewhere, but it is explicitly stated in Adventurer's Vault, p185 (the last paragraph before the chart starts) - though you have surmised the gist of it.
"The difference between a potion and an elixir is that a potion usually requires the imbiber to expend a healing surge to use its power, whereas drinking an elixir does not. However, consuming an elixir usually counts as a use of a magic item daily power; the same cannot be said for most potions."