From early to end game, bane/purge are always a very bad option.
Max blood oath (by far the best option)
Then either get to lvl 20 (after +skills) demonic mastery, or dump points into consume (if playing high player count or for ubers) or defiler (if playing low player count).
On budget, you could benefit from a point in hex:siphon, but this is soon irrelevant.