@Darren: I wrote about how to identify the most frequent word among several options here:
Using the resources mentioned in that post, I found that, for subtitle-based frequencies...
Sandwich is word form # 11759 (more common)
Emparedado is word form # 17635 (less common)
In the RAE list...
Sandwich is word form #34917
Emparedado is word form #63959 (twice as rare)
This difference between the subtitle-based frequency and RAE frequency tells you that "emparedado" is more often found in movie subtitles, not in actual speech, and even in subtitles, sandwich is more frequently used.
Using RAE's corpus search engine reveals that most examples of "emparedado" come from Spain, but many are not in the meaning of "sandwich", but in the meaning of "being between two walls", or "feeling trapped". Searching RAE's corpus for emparedado in the Americas returns very few results, except for Argentina. In my country, it returns none.
My personal experience: I have never heard anyone use "emparedado" outside of movie dubs and movie subtitles. It seems to be one of those words that translators use just to avoid using anglicisms. In my country, we say "sánduche", but this word might be considered "uneducated".