Well, saying heatercore vs radiator is kinda confusing, because a heatercore is just a type of radiator. I assume you're saying heater core vs something like a Black Ice...
If you can, go with a heater core. They've been shown to provide better performance, and don't restrict flow as much. Also, they're much cheaper and retailers usually sell shrouds with them. Their only problem is their size- if you're cramped for space, go with a Black Ice-type radiator. You'd still need to shroud it, but I don't know of any sold for these types of radiators, so you'd have to do it yourself.