: if I remember correct - the 500 is 2" in diameter - way much more then
: the 1.5"
: of the 400 . I believe it's thrust is twice as much (or so), so it takes half
: the time (or so) to empty a given volume - a quality that to my opinion
: is more important then the strength of the spring.

: any other opinions ?

The nominal diameter of the inlet and outlet piping of the 500's is 1.5", while that of the 400's is 1". The pump bodies are identical; only the inlet and outlet ports are different. The 500's have a slightly higer pumping capacity, but making that 1.5" hole in the deck look good is more difficult than for a 1" hole.

For me, the specific rate of pumping is not as critical a factor as that the footpumps allow the paddler to empty the boat while paddling, thus regaining control of the boat.