The improvement(s) to follow as I see them
Mechanical advantage (most important):
The purely lateral/outward force encountered on the 91 style extractor spring is changed in the E series to a "offeset" force where a lever (from extractor tooth to the pin) makes the force smaller on the spring/spring can impart a larger force to the extractor. Also the "hook" on the end of the E extractor acts as a stop for the extractor as well at full extension. This means on a 91 extractor, the spring will take 100 percent of the outward force if you have a jam and a E spring has a limit to what the spring will take, preventing permanent deformation of the spring. The pin will break before the spring will encounter deformation.
The spring design is also significant, the flat/cantilever style 91 spring is not as strong as a E compression spring when comparing equal sizes, even when the additional lever force is applied. The smaller coil spring will last longer than a flat spring.
Lastly the additional material on the E series face allow it to be stronger where the rim of the cartridge strains the extractor. I'm sure the metal is better as well but don't know the specs.
The E extractor works better dues to mechanical advantage, that prevents the failures in the 91 series. HK should have made all their bolts this way, or at least retrofitted them. The MAD bolt is a good example of how great the advantage is.
ETA crappy picture (not to scale/numbers are a guess) F=extraction force