When installing a sub panel in a single family dwelling coming off a main panel do you size your wire as a feeder according to 310.15 (b) (6) or do you size it as a branch circuit?

Depends on location.
In my area we are allowed to use 310.15(B)(6) for 100A sub-panels. Meaning #2al or #4cu.
I use #2SER ALL the time for 100A sub-panels.

And NO, I don't want to get into the argument about whether this is allowed. Our AHJs allow it and that's all I've ever known around here.

IMO it is common sense. In a residence a 100A sub-panel will have JUST as much load diversity as a main 100A panel, so why should a sub-panel be required to be fed with larger wire?

Anyway, check with your local AHJ and see what he says.
Worst case scenario, use a 90A breaker.

As Stran states, there should be no reason not to use table 310.15 (B) (6), as this tables is for residential services and feeders, and a cable to a subpanel is considered as a feeder.