We are buying a home in St. George, UT. Temps here range in the 100+ during summer. Is it better to have R15 and R50 or radiant barrrier?

The more insulation you can get, the better the structure will be at resisting heat gain & loss during the winter. The concrete tiles actually act as a radiant barrier if they allow for air to move under them (yes that is one reason why Spanish tiles were do popular in southern climates before they knew of building science). In this case the air gap allows for air to flow & the tiles provide shade for the attic essentially creating a cool roof assembly. As for radiant barriers, while they do have a place they are not a be all, end all - it only deals with one source of heat loss/gain: for more on this: http://thehtrc.com/2012/building-science-radiant-barriers As for how much insulation - there are numerous thoughts on this including payback - personally I say model it & make your decision based on what your goals are. It is always easier & cheaper to add more up front than to try to add more later