Update (just inside the 45 day limit for thread Necromancy).
I have decided to opt for an invocation that adds to battlefield control, moving enemies about the field. Usually I'd choose Pushing Blast for best Battlefield control, since you can push with every blast in a turn.
I'm also going to take Spirit Guardians as a bard 10 Magical secret. and I was thinking the I really need to be able to pull bad guys into SG for extra damage on my turn as well as on theirs when they start in the SG area.
So it seems obvious that adding Grasp of Hadar to pull bad guys 10 ft automatically when I hit with an eldritch blast (once per turn) would be ideal.
Grasp of Hadar wins for its synergy with Spirit Guardians.