It depends on the type of plastic and rubber. It also depends on the quality of the balloon. It would be hard for us to say that one type will definitely pop before the other type, because things can also be added to either rubber or plastic to make the balloons harder to pop, or make them different colors, and so on. A good way of finding out would be to find one of each type, note the material and any additives (things added to the balloons), and start putting air into these two types of balloons at the same rate - using a gas cylinder would be better than blowing them up by mouth because by mouth, it is much harder to keep the same rate of gas input into the balloon - and observe which balloon pops first by measuring the size (radius, circumference, or some other way).
Hope this helps!