I'm thinking mines. Just because I feel like the support weapon + backpack slot is already pretty crowded. I don't see a rocket backpack taking away my love for the Quasar and Shield combo.
Rockets seem more useful to me, since I can't count on tanks going where I want.
On the other hand, if the IRL combat footage videos are any indication, there might be black comedy potential with the AT mines I'm overlooking.
Elites will either go after who attacks them, and/or whoever is closest. I would say it is pretty easy to bait them into a minefield, if it had lower cooldown times than the railcannon and laser it might well be a great pick
Mines. I think they'll be handy if grunts/helldivers can't trigger them.
Rockets are cooler though.