Hard to do a direct comparison, because it is hard to drink liquid butane, and frostbite is the most common injury listed for contact with the liquid.
Ample evidence that huffing butane kills brain cells, but it is a function of displacing oxygen as a simple asphixiant, taking a 65% butane vapor atmosphere to kill half the rats.
Butane is no more toxic than Ethanol, and from a toxicity stand point, take a look at the effects of some of the naturally occurring terpenes in sufficient concentation.
That leads to the word concentration, which leads to the word perspective. Drinking most solvent, is an insalubrious act, but in parts per millionth, the issue is mostly moot. For instance, Methanol is the byproduct of fruit pectin fermentation, so is present in most orange juice at low levels. Our bodies have no problem processing methanol in small quantities, and evolved to do so.
If you look at the Ethanol molecule, you will see why they call it a simple Alkane alcohol. It is simply an Ethane molecule, with one oxygen atom added.
Butane is the easiest to purge to low residual levels, without losing the monoterpenes, because it has the lowest boiling point and is a simple non polar unsaturated alkane. You can also taste it at around 10% of the 8 hour PEL, so if you can't taste it, your exposure from a dozen hits is not only a small percent of PEL, but an even smaller percentage of an 8 hour exposure.
Note from the attached that the ACGIH TLV TWA: 1000 ppm 8 hour(s) for butane is for 8 hours and for ethanol only 15 minutes.
n-Butane
http://www.airgas.com/documents/pdf/001007.pdf
ACGIH TLV (United States, 3/2012).TWA: 1000 ppm 8 hour(s).
NIOSH REL (United States, 1/2013).TWA: 1900 mg/m³ 10 hour(s). TWA: 800 ppm 10 hour(s).
OSHA PEL 1989 (United States, 3/1989).TWA: 1900 mg/m³ 8 hour(s). TWA: 800 ppm 8 hour(s).
Ethanol
http://www.airgas.com/documents/pdf/001114.pdf
ACGIH TLV (United States, 1/2009). STEL: 1000 ppm 15 minute(s).
OSHA PEL 1989 (United States, 3/1989). TWA: 1000 ppm 8 hour(s). TWA: 1900 mg/m³ 8 hour(s).
NIOSH REL (United States, 6/2009). TWA: 1000 ppm 10 hour(s). TWA: 1900 mg/m³ 10 hour(s).
OSHA PEL (United States, 11/2006).TWA: 1000 ppm 8 hour(s). TWA: 1900 mg/m³ 8 hour(s).