Any water you drink goes towards the "8 glasses" thing, regardless of tap, bottle, mineral etc.
It's the minerals in the mineral water that make it better / worse than tap water.
I only say "worse" as some people who drink HEAPS of mineral water with VERY high mineral content can take on too many minerals and can cause kidney problems. I've only heard of this very randomly, so not a regular occurrence from what I understand.
I'm with YandiGirl and prefer sparkling mineral water and love the Mt Franklin one which is just Mt Franklin with bubbles, so doesn't have the high mineral content like other mineral waters. (Which is purely a flavour thing for me, I am not a fan of the taste of the more minerally waters