I agree with Aurelien about wind sensitivity.
The P-frame provides a rigid handlebar independent of the saddle and storage space up front. But I tend to think that you shouldn’t think too big for the surface area of the pannier.
I’m very happy with mine, but a smaller pannier might be more interesting for that reason. It’s not the volume that’s most interesting here. What’s interesting is the fact that you can fit the dense, heavy stuff you want to carry (water, tools…).