In the limited internet-based research I've done, I gather that the difference between Iso-propyl alcohol and Denatured alcohol are pretty basic.
Denatured alcohol is the same ethanol found in beer, wine, and spirits, but poisonous and foul tasting additives (like acetone, methanol, and sometimes even gasoline) are put in it specifically to keep people from drinking it. According to a doctor who posted to one U.S.Government website, some of these additives can cause blindness or possibly death if you're macho enough to drink the stuff anyway. Because of these additives, denatured alcohol is not reccommended for prolonged skin exposure. Hope you were wearing gloves when you filled your RC car's fuel tank. Denatured alcohol is generally used for cleaning and sterilizing medical equipment, and not on the patient.
Ethanol makes a fabulous fuel because of its volitility. There are special camping lamps and stoves designed to burn ethanol (or denatured alcohol,) and it is also the basis for some of the alternative fuels we see being pushed for use in our cars.
Iso-propyl alcohol is synthetic alcohol that is specifically formulated to be safely used on the skin. It is not ethanol, and will not make you drunk, just sicker than a dog if you try to drink it. It's also dilluted with water to about 70% strength, which might explain why it's a rotten fuel for RC cars and such.
Sources:
http://www.newton.dep.anl.gov/askasci/chem00/chem00102.htm
http://sci-toys.com/ingredients/alcohol.html