The answer depends somewhat on the hunter's age. The eye pupil dilates to a smaller size for people above the age of 50. Someone in his sixties will probably have 5-6 mm pupils in low light.
For some one with 6 mm pupils, both scopes will have the same brightness as the naked eye up to about 9.3 magnification. Above 9.3 the 56 mm scope will start to loose brightness. The 72 mm scope will have the same brightness up to 12 mag, above which it will also start to loose brightness. By comparison, a 40 mm scope starts to loose brightness at 6.7 mag.
In low light the human vision system trades off resolution against contrast. Higher magnification is like having higher contrast. In low light, contrast is what determines how well objects can be seen. The 40 mm scope allows someone with 6 mm pupils to see 6.7 times better than the naked eye, the 56 mm scope is 9.3 times better, and the 72 mm scope is 12 times better. "Better" can mean either higher contrast at a constant resolution (or range) or higher resolution (or range) at a constant contrast.
The phase of the moon, weather, terrain and vegetation are factors too. A quarter moon provides more light than clear starlight, which provides more light than overcast starlight. A flat prairie grassland provides more light than a tall forest at the bottom of a canyon.