I don't notice a difference between 3 gallon and 5 gallons whether its soil or hydro. I don't think autos grow enough size to necessitate a 5 gal.
But, if you are growing in soil with fabric pots definitely go with the 3 gallon.
A 3 gallon fabric pot with a diameter of 10" and a height of 7.5" has a volume of 589 cubinc inches with a surface area of 235.6 inches squared
A 5 gallon fabric pot with a diameter of 12" and a height of 9" has a volume of 1074 cubinc inches with a surface area of 385 inches squared
When you divide surface area by volume to get a ratio the 3 gallon has a surface area to volume ratio of 0.4 whereas the 5 gallon has a ratio of 0.33333333
The more surface area to volume means more oxygen absorption.