Technically Bill of Materials and Item Assembly are identical, where Bill of Materials already have assemble component item setup and you just pick how many you want to build  (on assembled inventory item record's Build tab have all component item require setup), where as Item Assembly, you have to pick the components and the Assembled Items to build.

so for frequently build inventory items better setup Item Assembly .

For that I would use Item Assembly, and save the first one as a recurring transaction.  Then if the others are slightly different you can edit the quantities as you go.

Bill Of Materials is best where you want to automate a little - i.e. you sell a 'filter kit' that's inventoried as a half dozen parts but sold as one item.

Item Assembly is better where the items / batches may be slightly different or use more or less labour.