For what I know about it.
R407C and R134a are both environmental friendly.
R407C is a replacement for R22
R134 is a replacement for R12
Now R407C is a mixture (blend) of three other refrigerants being R125 R32 and R134A in a mixture of 25/23/52 so R407C is for more than half R134A.
Now it’s property’s differ R134a not a mixture has got one evaporation point. R407C has not got an evaporation point but a range ( 400 series of refrigerants ).
A leakage in the system means that with R134A you can just fill it up again, but because R407C is a mixture you need to remove/ replace all because of the difference in evaporation point of each component. Although some debate this. ( google this on the net and you get plenty of info).
The pressures for R134A are indicated as middle, but the pressures in R407C are indicated as high.
Some people say that the lower pressure of the R134A is advantage over the higher pressure for R407C
This was my two cents.