Filia:
It depends - obviously if you need a get-off-me assist you should use H Updo, but Hairball is good for pressure or confirming into combos. Those are the two most everyone uses, although I have seen some cr.MKs.
Big Band:
I personally use Beat Extend L, because it his faster higher up than H. The other go-to assists are H Brass Knuckles or M A-Train, but for rushdown Beat Extend is probably the best choice.
Throw is also way better than people give it credit for being.