I lean towards Bernie because he has the better track record. However, if they are both polling behind Biden by the first primaries, I think Bernie will drop out in order to boost Warren.
As for the debates, the first night Warren and Sanders were excellent and Buttigieg was decent, but the rest sucked and should drop out. Bernie and Warren finally took off the kid gloves and hit back at the other candidates. The second night was a little more bland. Biden, Booker, and Harris should be in the next debate, but the rest should drop out.
Beto needs to run for Senate in Texas and start doing that soon. He could have a major impact of helping to flip the Senate. The VP pick won't make much difference in the general, and the VP in general doesn't matter.