Lt. Max said:
A radiator and heater core do essentially the same job, they transfer heat from a liquid to the ambient air. As for design and performance in the automotive realm, a heater core is compact with a very tight fin spacing and needs a high performance blower to get air through it effectively. On the other hand, a radiator is much larger, with more open fin spacing and flows air better and can use a larger axial fan (or 2).
It is a trade-off of size vs. air flow vs. performance. Ultimately, the air flow is almost always the restrictor on performance, hardly ever is the water side flow a restriction for cpu cooling. The questions to answer: 1) how much room do you have for either heat exchanger, 2) how big / loud of a fan or blower can you tolerate?
Good Luck.
Koooler King