Garbage disposal and dishwasher wiring

When we moved into this house, the batch-feed garbage disposal (thus with its own integral switch) was wired into the adjacent dishwasher's junction box and fed from a 20A breaker -- and, yes, with 12ga Romex.
Now we are remodeling our kitchen, and I want to have the dishwasher and garbage disposal (a new non-batch-feed one that needs its own switch) on separate circuits (which I understand the current NEC requires anyway).
Would it be OK to run an Edison circuit (replacing the original 2+ground wiring by 3+ground and fed from tied breakers) to a split receptacle in the wall behind the dishwasher, with the outlet for the garbage disposal controlled by a separate momentary-contact switch in the wall above the countertop (and separate from other switches and outlets to avoid confusion)?
When using 2+ground Romex for the switch, which conductor should be used for the "permanently hot" -- the black or the white? Or doesn't it matter?
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