Personally I am somewhere in the middle on this myself. I see the point being made by Elli and Haris (I had forgotten that Haris joined in on this, and I am really glad Nate reminded me of that. It is helpful validation to have TWO Greeks who are proficient in English to make the same observation) . I tend to think that the Greeks sense something deeper going on, and that the English ear is being tuned by something in the background that needs to be identified and resisted. (Yes, I can feel my "all Brits are Stoic" antenna coming into play even though that is my own heritage.)
But Joshua's point is correct too - In normal ordinary conversation I think that most English speakers (at least Americans) probably consider the terms "Epicurean Philosophy" and "Epicureanism" to be absolute equivalents. For that reason the effort to distinguish the two rings somewhat false and weird in American ears.
But maybe this is an area that the English-speakers have by force of habit accepted a viewpoint on "isms" that ought not be accepted in the first place. Even though the"ism" suffix does have a negative connotation in English, it is used interchangeably as designating any system --- perhaps that is part of the problem? Why use a negative term as an equivalent to a term that ought to be positive or at least neutral? (Is it possible that we have absorbed some kind of British cynicism / eclecticism that we need to root out and trample underfoot?)
Maybe any use of an "ism" ought to have an absolutely negative connotation, as it appears to have in Greek, and any use of that suffix ought to be clearly understood to be used as a denunciation.
Now the fact of the matter is that most Americans won't understand that point immediately - but maybe that is something that we all profit if we point out to them.
So while I agree with Joshua that the effort to distinguish sounds weird in English, I do think there is an issue here that can be helpful to address.