In the testing I have done pellets are usually higher in iron. Processing likely has an impact there. The amount was not huge though, and through r/s/r the iron content can be brought down to safer levels.
Pellets usually also have more molasses. This also is soaked and rinsed out.
The biggest reason I use the shreds is they can be made ready faster. Pellets take longer to soak than shreds.
Introduction is usually not an issue.
Nancy C in NH
ECIR Moderator 2003