OK, I'll take a more serious stab at the question. I imagine that they both taste like medicine, but I'll be a bit more specific than just that.
Health potions I imagine being like slightly thin cherry-flavored cough syrup with just a tiny bit of iron-tang. It's cloyingly sweet with that impossibly false-tasting flavor, but even all of that can't cover the fact that, carrying the magical essence of life itself, there's something about it that's inescapably reminiscent of blood.
Mana potions are different. First of all, they're of a thinner consistency and the artificial flavor isn't cherry. Maybe it's pineapple, maybe it's grape, but it isn't cherry. It isn't any better, though, and it's still got that unpleasant sweetness to it. The iron tang is absent, but in its place is a strange sensation you get when it enters your mouth. It feels thicker than it is -- slimy, almost, even though you know it isn't -- and it almost seems to seep into the wet epithelial tissue of your tongue and throat as you swallow. It's not so bad on your first go, but with every additional potion you drink, it feels thicker and more invasive until, eventually, downing the same quantity of potion -feels- like you're drinking at least three times as much.
That's my take on it, anyway.