battling ampersands

Abercrombie & Kent, a travel agency that uses a Times Roman ampersand as a trademark, sued andBEYOND, a travel agency, not for the tacky capitalization but for using a Gill Sans ampersand as a trademark.

I doubt that such a suit would succeed if the marks in question were very different graphic treatments of the same letter, say an angular S forming a thunderbolt versus a more stolid sort of S in a ring. How different would the newcomer’s mark need to be, and is the necessary difference greater for quasi-letters such as ampersand?