I've aquarium salted my way through some bonked scales and minor fin rot, so I wouldn't *not* have it. The salts are pretty much the cheapest things in the arsenal of betta remedies.
In the case of this thread, I'd have to ask...do we know what the life or death issue is? It seems like most of the things epsom is most helpful with have greater potential to be lethal faster than the things AQ salt is best to treat. So the epsom is probably more potentially lifesaving.
I have both. I got my IALs from Amy Lin last month. I just bought the two Maracyns so I have them available if I ever need them. Next on the fishy first aid kit acquisition list will be some of those Jungle Cure fizzy tabs.
Prepared is good
The Boy Feesh: Indigo CT, Lavatail CT, Ghost CT, Chicory HM, Redfish VT, Magic DT
The Girl Feesh: Christmas CT, Shine VT, Snowflake CT, Stripes VT, Midnight CT, Grif VT, Tangerine VT, Wraith CT, Amethyst VT, Ice CT, Poe CT, Turquoise II CT