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Saguenay's fjords aren't at all like either Cape Breton Highlands or the coast and mountains of southwestern British Columbia. I wouldn't view any one of them as being at all close to being a substitute for the other:
Further up the B.C. coast there are deep inlets that are really fjords, but even then, the terrain is different and the feel of the place is different.
One's opinions about scenery are very subjective, and will vary greatly from person to person, but if the Saguenay region looks appealing, I'd say go for it and visit it. There are also some impressive areas of Quebec further downriver on the St. Lawrence. Plus, as you mention, the culture of Quebec is very different from anywhere else in Canada.
To really differentiate them, I'd need to know your personal preferences, and we shy away from customizing advice for individuals here, preferring rather to give content that will be of benefit to a broad audience.
Look up information about each area online and see if there is an appeal to visit them. Frankly, even within very similar-seeming areas, there are significant differences to those who pay attention. For example, most lump the four Atlantic Canadian provinces together, but I've visited all four and have different opinions about each of them. The people are different, the scenery is slightly to significantly different, the feel of each place is different. Some places I appreciate because they are off the beaten track; others I appreciate because they are so gorgeous that the entire world knows. For example, one of the most popular places to go in Nova Scotia is Peggy's Cove, near Halifax, but I enjoyed our drive down a chain of islands to Westport just as much, and it was bereft of tourists.