Fwiw, I just watched this mini review by chance. Its shows some web surfing benchmarks by cinebench (never heard of it before). And it say the PRO comes out with 25% more battery time than the Air. At 1m 42sec but only 4m vid.
Pro - 1223 mins = 20 hours
Air - 960 mins = 16 hours
So
4 hours more for the Pro, which is 25% more battery time for web surfing than the Air's 16h.
This is quite a lot I think. Worth it for $300/£235?
You will be charging the Pro less often and also the battery will have less cycles and therefore hold value better, theorectically.