R503: blend
CFC-503, a blend refrigerant of HFC-23 and CFC-13, used in Iow tempreture frosting unit.
Typical Properties:
Formula: CHF3/CCIF3
Boiling Point (101.3KPa, ~C): -87.8
Freezing Point (~C): --
Critical Temperature (~C): 19.4
Critical Pressure (KPa): 4357.5
Liquid Density (kg/m3): 1233
CDP Value: 0.6
GWP Value: 13.85


R-500: blend
R-500 is a blend refrigerant of CFC12 and HFC 152a. It has higher pressure than CFC12, therefore under the same compressor's capacity, it's more efficient.
Typical Properties:
Formula: CCl2F2/CH3CHF2
Boiling Point (101.3KPa, ~C): 99.3
Freezing Point (~C): -159.8
Critical Temperature (~C): 105.6
Critical Pressure (KPa): 4426.5
Liquid Density (kg/m3): 1138.9
ODP: 0.7
GWP: 2.1-2.5


R-404A: blend
R-404A is a blend refrigerant of HFC125, HFC-134a and HFC-143. AlliedSigal Inc.'s patent number is AZ-50. It is a long life substitute of R502, harmless to Ozone. It is mainly used in Iow and middle central commercial air re-frigeration system.
Typical Properties:
Formula: OH F2CF3/CF3OH2F/OH3CF3
Boiling Point (101.3KPa, ~C): -46.1
Freezing Point (~C): --
Critical Temperature (~C): 72.4
Critical Pressure (KPa): 3688.7
Liquid Density (kg/m3): 1017.2
ODP: 0
GWP: 0.35


Info found at this link http://www.favorcool.com/refirgerant.htm

What makes one better or worse than the other?? Price, avail, danger levels, usability, boiling points??

thanks for your input