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?).
Example: Potion of Fonzie vs Elixir of Cool (because I daren't use actual WOT examples.)
Potion of Fonzie
Power (Consumable): Drink this potion and
spend a healing surge. You do not regain hit
points. Instead, gain a +5 on the next skill
check that uses your Charisma modifier.
Elixir of Cool
Power (Consumable): Minor Action. You
drink the elixir and gain a +5 to your Charisma
modifier for all skill checks, (but not for
attacks) until the end of the encounter or for
5 minutes.
Special: Consuming this counts as a use of
a magic item daily power.
-Arlough the Wonderfuzz
"I've never actually been poisoned. I've been drugged, addicted, diseased, and set on fire a few times, but never poisoned." -Icemoore Riogale