For starters, a 40 gallon breeder is not 40 gallons and a 50 gallon breeder is not 50 gallons. Tanks are just sold as a ___ gallon for a rough guideline.
You're right, WPG is not the best way to evaluate lighting as this very situation shows. My opinion is that footprint matters more than gallons. Yeah, not if you're comparing a 12 inch tall tank to a 28 inch tall tank, but in general.
For either of these tanks, I would say that 96 Watts is enough light for low to medium light loving plants, but it may not be good enough coverage.