Ok I'm about to go on a bit of a tirade, bare with me....there are some functional differences between ethanol and isopropyl alcohol when it acts as a solvent. In ethanol the carbon chain is stright with an alcohol group on it (OH) where as isopropyl is not a straight chain it looks more like the inside of a peace sign with an alcohol group attached to it. However with that said, for the purposes of cleaning out a pipe, atleast in practice they both seem to do fairly well and they act very similar (without knowing the composition of what makes a dirty pipe dirty, I wouldn't know exactly tho) personally I use isopropyl (I try to find as close to 100% as possible) because I know it has a fairly low evaporation point and they usually dont add anything extra in it. I would NEVER use denatured alcohol in my pipe. The whole reason it's called denatured is because that means they have added something to it, like a bittent or other fowl chemical, to make it no longer good for human consumption.... And get some of that "bitter x" in your pipe forget about it. It's so bitter its detectable in concentrations of something like 1 ppb. Might as well toss that one in the fire.