It's very tempting to immediately go with b), but experience has taught me otherwise, especially when it comes to colour film where multiple stops overexposure will affect colour fidelity.
Some leaf shutters max out at 1/250th (e.g. the Konica Hexar AF). Being forced to shoot everything at F22 in bright light (not exactly a lens's sweet spot) becomes tiresome really fast (been there, done that). So, no matter how well I know how to expose a particular 400 ISO film, anything 100 ISO that I've never shot before with will allow better results. Even the Bronica, which officially can go 1/500, but in reality does 1/350th, having 400 ISO only would force me to shoot F16 or up in bright light.
So, it's not so clear cut between a) and b)..