In the end, it doesn’t make much difference whether the nominee is a realo or a fundi; the only meaningful legislation that will get through the system will be a rollback of the Trump tax cut and some corresponding new spending, and even that will only happen if the Democrats take back the Senate. Don’t bet the ranch on it.
Having said that, a fundi president will probably blame moderate Democrats for his/her inability to bring about the revolution, thereby sparking an intraparty civil war. In addition, a President Sanders would let loose all sorts of evil forces throughout the world as a result of his unwillingness to do anything other than talk in any circumstances. Bernie is consequently the worst of the available Democratic options.
By contrast, a second term for Trump will let loose the beast to an even greater extent than we see today. Together with his sidekicks, Bolton and Barr, there is no end to the damage he could do, both domestically and abroad. Even Bernie would be better than that.