The first thing to remember is that most people are, unlike people who join or work on campaigns, not political junkies. So most prospective voters are really not aware of major policy differences between Warren and Sanders. Many think of Warren as a younger female version of Sanders.
I recommend arguing on a strategic basis. With his strong support among voters under 40 and/or earning less than $50,000/year, Sanders is the only candidate among the top three who can expand the voter base. These voters are intermittent voters, and a few of them staying home in 2016 reduced voter turnout just enough to cause Clinton to lose key Rust Belt States. (Remember that Nate Silver gave her an 89% chance of winning.) Biden’s support is mostly about name recognition, and Warren’s base of support skews heavily toward white affluent professionals. Not that there’s anything wrong with that, but those people are already likely to be consistent and frequent voters.
In 2016, Sanders got more than twice as many votes from voters 18-30 than Trump and Clinton combined, and the electorate is even younger this year. Also, Sanders appeals strongly to independents (40% of the electorate). In 2016 63% of them (of those who were allowed to vote in Democratic primaries) supported Sanders.
Jesus: Hey, Dad? God: Yes, Son? Jesus: Western civilization followed me home. Can I keep it? God: Certainly not! And put it down this minute--you don't know where it's been! Tom Robbins in Another Roadside Attraction