I switched my ferrets to Science Diet from Iams and haven't noticed
any difference in terms of coat thickness\texture or energy.  However,
the SD pellets are smaller and round, and the Iams pellets are larger
and used to occasionally get actually jammed into the top of Antigone's
mouth, to her great distress.  The first time this happened, I found
her hopping around on the floor and pawing at her mouth, without any
success.  I had to (gently) dislodge the pellet for her, so I decided to
switch.
 
Victoria (overworked human)
Antigone and Minima (fat and lazy "geriatric" ferrets)
 
[Posted in FML issue 0717]