Is there one phone card in particular that is more widely accepted by the public phone machines and/or cheaper than others for calls both in Italy and to the US?
This isn't really a direct answer to your question, but I've always just asked for a phone card from a newsstand upon arrival and have found them to be much cheaper and easier than anything I've bought ahead of time from the US!
I've also used the PIN cards that you get from the tobacco/news shops. Buying one for a specific country, Italy, works better and is cheaper than a multi-country card. In my experience, they don't work in every phone. I'm not sure if it's the phones or the cards. I usually have the place where I buy the card point to the closest phone I can use then look for a logo or something to identify the phone with the card.
I've had some good experiences with internet cafes for long distance calling. If there is one convenient to you, you might check out that option.