Isopropyl Alcohol as others have said is synthetic and usually a 70/30 mixture, the 30% being water.
Denatured Alcohol (Ethanol) is considered a purer alcohol at 95%-99.5% content.
The main difference is that Iso is considered safe for use externally in medicinal purposes (aka Rubbing Alcohol) whereas Denatured is ethanol which has had a poisonous component added to it during it's creation to make it unsuitable for further distillation.
Some of the chemicals that ethanol is combined with to make Denatured Alcohol are: gasoline, isopropyl alcohol, tertiary butyl alcohol, etc. which can cause cancer, liver damage, blindness and/or death if consumed.
Long story short, they both work but Isopropyl is generally less harsh on skin (and leather) and has a somewhat smaller concentration of bad stuff. In either case, wear gloves, eye protection and work in a ventilated area.
Side note - I use Denatured as most commercially produced dyes are formulated with it versus Isopropyl. I find that I have better results by being consistent in the solvents to thin & clean, especially when I'm airbrushing.