That's not as clear cut as the discussion here implies. When I was a kid 30-40 years ago, indeed I never heard כריך except as a trivia anecdote ("did you know this word for סדנביץ'?")
But in last decade or two כריך is making bold penetration into spoken Hebrew. I think it attacks in two vectors: one is cafes, where the menu would almost always say כריך, and the waiters follow suit. The second vector is educationals institutions, where the caretakers started saying כריך, not only to children but also when communicating to parents. So even people who are used to say סנדביץ', once they have children start getting used to כריך. For me (a seven years parent) כריך is definitely the default now.
Well, English loan words containing double-u [w] should be written with Waw, therefore the word is best spelt סֶנְדְּוִיץ׳ (or סנדוויץ׳ without dots), but the spelling with Beth is not rarely seen for this word. PS Do not forget the Geresh which changes the pronunciation of Tsadek from [ts] to [tsh].