Either is fine. I’d prolly go 40breeder since it has a bigger footprint.
Even though I’m new I know enough to agree with thisI'm thinking bigger footprint would be better than bigger water. The extra room for equipment, refugium, etc. may end up being more important than the gallons.
I was gonna build stand a lil bigger to hold stuffThe only problem you might run into is fitting the 40breeder under the stand if the tank is 18” wide