I would go with the latter .. I have 2300 sq ft in MI with a 2 ton .. it should have been a 3 ...I am getting a new heat pump and 1/2 of the contractors recommend 2 ton and the other 1/2 say 2.5 ton. The condo originally came with 1.5ton.
I get lots of sun and the cooling is ok, not great.
Yeah that rule of thumb is pretty reliable in most cases... For instance, your 2400 sq foot home is only able to keep temps at 77 deg. With your 2.5 ton system... If you had 6 tons of cooling you would be able to maintain 70 degrees with better efficiency... And your ac wouldn't run around the clock... Which would also save you in maintenance costs for the long run.Heartbroke, interesting advice. Good to know!