arthurg: I've found that the castellano-speaking singers have a little more clarity in their voice as a rule than the Latin American singers. There are exceptions, but as a whole, the Spaniards seem to be clearer.
That having been said, I actually recommend an Italian, Eros Ramazzotti, when people ask me for some good "easy (to understand) listening" Spanish music. The upside on that is that it's not horrible to listen too, either!
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Ongi etorri!