The P100 has amazing double precision (FP64) flops (due to a 1:2 FP ratio that got nixed on all other cards) and a higher memory bandwidth which made it a really standout GPU for scientific computing applications. Computational Fluid Dynamics, etc.

The P40 was aimed at the image and video cloud processing market I think, and thus the GDDR ram instead of HBM, so it got more VRAM but at much less bandwidth.