Are there other properties besides lower boiling point that make isobutane a better refrigerant than butane? Asked differently, if -1C is low enough for the application is there any reason not to use butane rather than isobutane as the working fluid in a refrigeration system?
 A: They have different pressure curves:

But to answer the question: yes, there are other properties that make isobutane preferred over n-butane. The most widely
used refrigerants in household appliances are isobutane and R134a according to "Evaluation of N-Butane as a Potential Refrigerant
for Household Compressors" by Preben Bjerre & Per Larsen (2006). See the following quote:

The theoretical evaluation shows at LBP CECOMAF that N-butane (R600) has:
  • 2.8% higher theoretical COP compared to isobutane (R600a) 
  • About 70% of the capacity of isobutane 
  • 10% higher theoretical COP compared to R134a 
The measurements show that the higher theoretical COP of N-butane compared to isobutane can not be found in the
  measurements. This is caused by the higher sensitivity to clearance volume, suction gas pressure drop and heating,
  compared to isobutane. However the COP level of N-butane in the present compressor design is at the same level as
  isobutane, which today is the refrigerant giving the highest COP on the market.
  Lifetime test show acceptable results. The wear tendency is comparable with isobutane.
N-butane is an option for reaching high COP levels in household appliances, but it does not offer significant
  advantages to isobutane on existing isobutane stroke volumes. However it opens the possibility to extend the range
  to lower capacity by using the existing compressor designs. The disadvantages are that the cost and size are
  unchanged.

$\huge \text{Glossary:} $
LBP: Low Back Pressure. "The minimum evaporating temperature and the condensing temperature allows for the
identification of the compressor application (LBP, MBP, or HBP). Low Back Pressure
systems such as freezers have evaporator temperatures below -20ºC (-4ºF). " 
CECOMAF: European Committee of Refrigeration Equipment Manufacturers. Or, in French Comité Européen des Constructeurs de Matériel Frigorifique. 
COP: Coefficient of Performance. "For a refrigeration system a COP of 4 indicates that 1 kW of electricity is needed for a evaporator to extract 4 kW of heat." 
R134a: refrigerant code name for 1,1,1,2-Tetrafluoroethane: 
 
R600: refrigerant code name for n-butane: 
 
R600a: refrigerant code name for isobutane: 

