Cleon
Legend
I could go either way on that. There is something sensible about using the monstrous spider advancement, though.
Sensible? I didn't get where I am today by being sensible!

Maybe it would help if we considered the Bite damage. If we start at, say, 1d4, it would go:
Normal Monster Advancement:
Small --- Str 4, Bite 1d4-3 -(average 1)-- Base
Medium -- Str 8, Bite 1d6-1 -(average 2.5) +1.5
Large -- Str 16, Bite 1d8+4 -(average 8.5) +6
Huge --- Str 24, Bite 2d6+10 (average 17)- +8.5
Monstrous Spider Advancement:
Small -- Str 10, Bite 1d4 - (average 2.5 ) Base
Medium - Str 14, Bite 1d6+3 (average 6 .5) +4
Large -- Str 18, Bite 1d8+6 (average 10.5) +4
Huge --- Str 22, Bite 2d6+9 (average 16)-- +6
Of those the Monstrous Spider version gives a much more linear progression, while the normal advancement gives a sudden jump in damage from Medium to Large.
Bearing that in mind, I think I prefer the Monstrous Spider version.