My Love rarely says, "Eh?". Save from when she's telling that old joke, that is: "How do you spell Canada? C-eh-n-eh-d-eh." Now myself, being American and all, I say the equivalent, "Huh?" all the time. So what's the difference?
Is it what they mean?
Eh - According to the Wikipedia summary: The only usage of eh? that is exclusive to Canada, according to the Canadian Oxford Dictionary, is for "ascertaining the comprehension, continued interest, agreement, etc., of the person or persons addressed" as in, "It's four kilometres away, eh, so I have to go by bike." In that case, eh? is used to confirm the attention of the listener and to invite a supportive noise such as "Mm" or "Oh" or "Okay". This usage may be paraphrased as "I'm checking to see that you're [listening/following/in agreement] so I can continue." Grammatically, this usage constitutes an interjection; functionally, it is an implicit request for back-channel communication. "Eh" can also be added to the end of a declarative sentence to turn it into a question. For example: "The weather is nice." becomes "The weather is nice, eh?" This same phrase could also be taken as "The weather is nice, don't you agree?".(more)
Huh - This is described much more succinctly by Merriam Webster: used to express surprise, disbelief, or confusion, or as an inquiry inviting affirmative reply. (more)
Americans joke about Canadians, or imitate them by adding strings of eh to sentences, but the reality is that it's far more common in my experience to hear huh in America than it is to hear eh in Canada.
But maybe in both countries I'm simply hanging with the wrong crowd, eh?