RE: saving bounties to cash in on the 15th.
I was doing this until I realized two things. First, the ~eight or so story missions are probably designed to get us to 40 anyways, like the old campaign was designed to get us from 1-20. So there's not much point other than to hit a number sooner.
The other is the fact that story missions don't have the option of increasing the difficulty above the fixed level; there's the normal mode, and the much harder setting. Doesn't seem to be in between ones like before. So leveling up a few times, early, will just make you blow through the campaign that much more easily. If you're trying to power through for the raid, I suppose that makes sense, but it will make that first campaign romp a lot less fun, or at least, challenging.
Bonus #3 reason: the XP awarded from story missions scales to how over or under leveled you are. If you run a 34 story mission as a level 38 player, you're going to get a LOT less XP than normal. It will at least partly offset the gains from dumping bounties on the 15h.
At any rate, I piled up 10 of them, and then cashed them out last week when I realized I want to take that first campaign run slowly.
Unrelated, I noticed this last night. The tower on Mars bears a more than passing resemblance to the bases on Valhalla. Wish I could get a shot from that angle further back, as the tower height and general shape is similar to the spire from the MP map.