Does it really matter who the backup is? If Dak goes down for the season it wouldn't matter they wouldn't be winning the championship. If Dak was out for a couple of games they could win a game or 2 with either Lance or Rush. I would be fine with letting Lance play. That could work for a couple of games and get him some experience. They would run the ball a lot more with Lance at qb and control the clock.
They won't win a championship with Dak either.
But if he's out they're still better than a few other teams out there. I remember Dak fans stating the season was over when he went down two years ago. Then came Rush and they stayed right in the contest.
Post season results would be the same, one and out. And we'd have a few regular season wins less with Rush.
But, since Lance needs more practice I'd go with him even though at this point he's worse than Rush.
So my choice would be Lance despite a lot of probable losses. Simply because I'd like to see him in a couple of games with the starters.