I'm shocked that the Democrats think that using the same formula won't produce the same results, I'm starting to think that they don't care.
we actually have a lot of head to head polling data on trump vs the various candidates. it's not just everyone diving into their own head and trying extrapolate from their own thoughts how other people view the world (although that can be helpful, i'm not completely knocking it).
biden generally has polled a point or two better than sanders over the last year. that mostly reversed and they were about even over the last month or so when sanders overtook biden as frontrunner.
my own interpretation on that is that biden probably has a durable advantage of about a point, maybe a little less, compared to sanders and that there is a dem consolidation effect worth about a point that accrues to the frontrunner.
both are very well-known and have pretty defined public images, so while opinions would change if they became the dem nominee (dems would like them more, independents and republicans would like them less), there's no good reason to think one would change more or less than the other.
obviously, a one point advantage isn't all that much and either could win or lose.