The big differences are material/weight and tread integration (both have replaceable treads).
Material
IFI: aluminum
AM: polycarbonate - AM also sells aluminum
performance wheels with a different hub design than IFI. These are also lighter at each diameter than the IFI versions.
The IFIs are heavier than the AMs at every diameter (4", 6", 8") - difference increases with diameter.
I haven't noticed any difference in performance based on the material.
Tread Insertion
IFI: around wheel perimeter only
AM: inserted between the two wheel halves which are unscrewed by 6 bolts (fewer wheel-tread attachments needed)
Tread Attachment
IFI: rivets (only?), tread is simply riveted on the outside - this is a real pain for some team/drivetrains
AM: lacing or
screws - we like this
Width
IFI: 1", 1.5", 2"
AM: 1" only
Treads
I believe the treads each company sells may be used on the others wheels. We've done IFI tread on Plactions, though it's a little tougher.
IFI: IIRC, they're slightly thicker with like an extra layer of backing on them.
AM: Anecdotally, we feel they wear better and may have slightly higher CoFs.
They're sold differently in terms of bearing options and sprockets, but I don't think there's actually a bearing ID or a sprocket tooth count (#25 or #35 chain) that you can achieve with one but not the other. I may be corrected, though.