i have about 6 Soldering Irons, from 10 Watts up to 100 Watts.
the only thoughts i can give you would be;
it all depends on the thickness of the Brass.
with small, thin bits of brass, up to 0,5 mm i use the 10, or 15 Watt, up to 3mm a 20, or 35 Watt, and anything over that depending on the joint needed, the 50, or 75 Watt.
the 100 Watt i only use for making Battery packs, and soldering connectors onto Cables.