Foam Insulation vs. Radiant Barrier
Most foam insulations protect against convection and conduction transfers of heat. Radiant Barrier works against radiant heat that is traveling through the air space. Truth be told, you shouldn’t be without either, if at all possible, as they both compliment each other to effectively protect against heat loss and gain.
If you have to choose one over the other, foam insulation is probably the more important, particularly if you currently have neither. However, you should consider a radiant barrier product in the future because of its potential to save you money on your heating bills.