We're in the process of adding rigid insulation on the exterior of our roof deck right now with standing seam over it for a vaulted ceiling. If you can get enough R-value from just the mineral wool then that would be easier, but we're trying to work with an older house without as much depth as you have for the rafters.
If you do go with exterior then keep in mind that your roof deck will now be drying to the inside, so don't use poly under the roof deck (which it doesn't sound like you would be doing). Also, take into account how exterior rigid will change your dew point. The colder it is, the more rigid you need in order to keep the dew point out of your sheathing. We're close to Canada, so by spec rigid should supply 61% of our total R-value to ensure the dew point is kept far enough away. Also, polyiso was mentioned here, but a mix of EPS with ISO under it will perform better in extreme cold temperatures (if that applies to you) than just ISO since ISO can lose up to 25% of its Rvalue when cold enough. Some EPS above it will help it maintain a higher average temp and thus more R-value.
We'll also have a 1" channel between the rigid and the roof, so air can flow through that to give us a cold roof instead of this terrible hot roof setup (read ice dam and leak maker) that came with this 1930 house. The proof will be in the pudding, but I'm hoping for no ice dams this winter. Good luck over there!
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