an RO machine is expensive. i've considered buying one many times but was never willing to cough up the cash for one.
i pay about a $1.20 per gallon for RO water, which is pretty cheap.
nutes are expensive. my nutes (and i would imagine all nutes) have magnesium and calcium. But this way you have better control as to how much mag and cal and other crap is in the reservoir water.
in any case to each their own.
asiankate, these are mostly personal preferences. if you decide to use tap water, it'll work just fine(remember to let it stand 24hrs to let the chlorine evaporate). I buy RO water because it's cheap and readily available.