Harold I happen to like the circulator on the RETURN side of the boiler for the following reasons.
1- The return water is slightly lower then the supply and thus less thermo stress on my pump.
2-Hot water has a tendency to rise so the circulator only has to help pull the colder water down
3- If any air is in the system it will not be pushed through the system by a pump on the supply side
4- Its been like this ever sinse they took out gravity systems so old and proven ways still work as the “DEAD MEN” knew what to to and how to do it RIGHT