Fiat G.50 and C.200 serie 3 vs serie 7

I have unlocked both of the serie 7 variants, but I am confused. In SB they have a higher BR and in RB the same, but instead of being an improvement their stat cards are worse in terms of climbrate. The only physical difference is the addition of another fuel tank. Is that all?

More or less, yeah. They are separate planes in order to help flesh out line ups and can be entirely skipped over, however having more planes in your back pocket is never a bad thing.