1 point that has not been addressed in the weight comparison issue is the grade of steel. Modern production processes have allowed for lighter steels to be much stronger than other steels. This general comes with a price premium and thus will be pointed out.
Weight is a factor in the effectiveness of a BB, but only 1 factor.
While older implements tend to be heavier, they tend to not be quick hitch compatible without at least some modifying being done. This may or may not be important to you.
Lots of good info in other comments, but one last caution is to watch your implement and try to avoid abuse, it is hard on your implement and your tractor.