I believe I would choose the province with iron/metalsmith option over the province with a specific unit bonus. The weapon upgrade will apply to any unit that is trained in that province whereas the other only applies to one particular unit type.

Once you have trained enough troops with weapon upgrades, seize control of the province with a unit bonus. Train the appropriate unit to get the bonus and then send him on over to the province with the weapon upgrade and retrain him so he can benefit from the increased weapon damage. Best of both worlds