While both of these measures have their flaws, I would argue that gross domestic product (GDP) is the superior measuring device. I would say this because it is more objective than the genuine progress index (GPI) and because it measures things that are more tangible than things that GPI measures.
Of course, GDP has its problems. It does not measure the value of leisure time. It does not measure the value of work, such as housework, done for no pay. It does not account for the fact that some economic activity can be harmful to a country. However, all of the things that it does measure can at least be measured objectively.
One trouble with GPI is that many of the things that it measures cannot really have a dollar value placed on them in any accurate way. How much is vacation worth? How much does it cost us when we deplete wetlands? These are matters of opinion. In fact, a second problem with GPI is that it is very subjective. It relies on opinions and values to decide which things have value. For example, do we know for certain that it is bad when farmland gets used for other purposes? Do we know for sure that it would be better to have a society in which everyone had an equal amount of wealth?
GDP has its flaws, but at least it measures tangible things in an objective way. I would say that it is the better measure.