Help understanding carburetors - metering rods and jets

I am in the market for a new carburetor; looking at the edelbrock 600 CFM performer series. There are 3 models to choose from that all have differences; with one marked for fuel economy and another performance
The 1406 (fuel economy) seems to be the most popular (cheapest), has .098 primary with .095 secondary, 4” spring and .075x.047 metering rods
The 1400 has .098 primary with .095 secondary, 5” spring and .073x.047 metering rods
The performance model is the 1405 which has .1 primary with .095 secondary, 5” spring and .070x.047 metering rods

What performance differences does the larger step-up spring make?
And what difference does the smaller metering rods make? Does this limit the amount of fuel that moves through the jets? Or the rate of fuel delivery to the jets?

The 1400 and the 1405 with the electric choke kit are about the same price. And the thunder 1806 650CFM has .095 primary with .098 secondary, 5” spring and .068x.047 metering rods is just a little more expensive
Thanks for the education