Hello Alayne,
Not sure about the two kinds of marshalls, but it made my ferret's poop
extra stinky.  I think the fish oil has something to do with it.  But if
they like it and you don't notice a if difference, go for it.  As far as
learning about other ferret food brands, check out the ferretstore.com
 
They have a large variety and a really cool ferret foodchart analysis
that tells how much protein, fat and other ingredients each brand has
and compares each.
 
Personally I like to mix a whole bunch of different kinds together, since
ferts are so picky about trying new stuff.  This way if any of the brands
become unavailable, I still have a lot to chose from.  Mixing the new
kind of food in with the old is a good way to get them used to it.
 
I have a large sealable bucket, holds 20 lbs., that I bought from the
ferret store and I mix everything in it all together.
 
I usually use, Path Valley, Sheppard and Greene, 8 in 1 Ultimate, The
Ferret Store Superior Choice, Totally Ferret, and Mazuri.
 
They seem to like it.
Good luck,
suzi
[Posted in FML issue 4082]