The Nissens is likely aluminum with plastic side tanks. Your original must be copper? Guessing because they said it was rebuildable.
Did the original radiator ever cool better than it is now? If so, I would rebuild it to stock (if the price is reasonable) and keep it as a spare.
This is a surprisingly complex comparison, but with a little study you can understand it.
Here is my favorite reading material on the subject:
http://caparadiator.com/aluminum-vs-copper/