First off, what is a brass radiator?
Conventional radiators usually have brass top & bottom tanks & brass tubes. The tubes pass through copper fins which are soldered to the tubes where they pass through the holes in the fins.
Brass is structurally stronger than copper, thus it's use in the tanks & tubes. Copper has more thermal conductivity than brass, thus it's use for the fins.
If "brass radiator" means that the fins as well as the tubes are brass, the only possible advantage would be more mechanical strength. The down side would be less cooling effeciency for the same radiator size & layout.
It appears that with either type fins, the fin to tube solder bond deteriorates over time, reduces heat transfer, & reduces the cooling effeciency. Because of this, a new radiator of the same exact materials & construction will often give better cooling.
You could ask DW why they are offering "brass radiators" & what?
To ramble on a bit:
Conventional radiators of soldered construction have become an environmental problem. With millions of radiators to be disposed of, the considerable amount of lead used in the soldered construction is a "hazardous" material that creats disposal & recycling problems.
To combat the environmental problems, to reduce weight, & to reduce costs, most manufacturers have switched to aluminum radiators which don't require lead solder. Additionally, the top & bottom tanks are often plastic which is mechanically clamped to the tube assembly.
There are some high quality after market aluminum radiators available which are of all braised construction, lighter in weight, & very sturdy. Because aluminum has lower thermal conductivity, aluminum radiators must have quite different tube designs, better fin designs, & generally much different from their brass/copper counterparts.
D