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What is better for a SM/IG gunline?

Depends on what the rest of your list is like, doesn't it?

If you're lacking anti-horde, go TFC.
If you're lacking anti-MEQ/vehicle, go Basilisk.

Which force is going to be the ally?

Defiantly a thunderfire cannon, it's anti-infantry ability and trick to boost the save of terrain is a great boost to the army as a whole.

This isn't really a plausible question. For example, if the Space Marines are the main detachment, then the real question is, 'Thunderfire Cannon or Basilisk + HQ + Troop'.

You can not fire directly within 36'' with Basilisk. That would be the deciding factor for me.

Though as stated they have different roles and also act differently, one being static, one not.

Crazy question. Basilisks look cooler?