Here's what that very page says about the ACR compared to the Mini-Fox 40. I quote:

"The reason we really like the ACR Emergency Whistle's flat design, however, is that it's virtually unnoticeable when hanging off a lanyard (included) around your neck, unlike bulkier whistles like the Fox 40 Mini Whistle.

We compared the performance of the ACR WW-3 Emergency Whistle to the Fox 40 Mini Whistle (another backcountry favorite), and here's what we found: The ACR Emergency Whistle requires less air to blow at a particular decibel level than the Fox 40 Mini Whistle. In addition, the maximum db output of the ACR Emergency Whistle is greater than the Fox 40 Mini. We think this performance, combined with the flat, lay-next-to-your-chest-and-not-be-noticed-design makes the ACR Emergency Whistle the ideal whistle for the backcountry traveler."

That would seem to answer some questions. I've seen an ACR in a store and it offers a much flatter profile in comparison to the Mini 40.

-- Craig