Help understanding carburetors - metering rods and jets

The 1406 is an electric choke model
1405 is the manual choke model
They are the same carb otherwise.

Step up springs are what determines how much fuel is allowed through the jets at any given throttle and have no specific bearing on performance or non performance, as they come in different strengths (poundages) that determine how much the needle rises out of the jet under different vacuum levels. (caused by throttle position)

I would call the 1405 and 6 as nice reliable daily driver carbs and the Thunder series as a more performance oriented carb.
The electric chokes on the 1406 series work perfectly, and are nice for a daily driver.
If you want more control over the choke amount, duration and warmup RPM's go with the 1405 for the manual choke.