Actually, inflation was only part of Spain's problem. When the whole of the Hapsburg money making machine is looked at, Spain was one of the most profitable parts of it. The problem for Spain was that its tax money, rather then being spent on its own improvement, went on fighting expensive and largely futile wars elsewhere in Europe, like the Netherlands. Perhaps if that money had been spent differently, Spain could have gotten much more reward for it.
Just as inflation in of itself was only part of the problem, so to are those wars. Not one is significantly more important than the other; Hell, France was able to put off their war debt for two hundred years... The breakdown of Spain looks like this:
1. Huge amounts of unregulated monies from the new world = Inflation for the citizens, making it hard for an Industrial base to form in these times.
2. Spain, with much of their wealth, buys stuff from other countries, strengthening their enemies and making an industrial base (and through so a middle class) even more difficult.
3. The Alhambra Decree of 1492 evicted all Jews and Muslims, which were the economic backbone of Spain at the time, and without them the early merchant-middle class was virtually destroyed, as well as the institutions of banking and interest.
4. The Catholic church was the largest employer in Spain at the time, meaning a large amount of childless men, meaning a depression in population. The Church was given tons of monies to facilitate things such as cathedral building and the inquisition, which really didn't do much to benefit Spanish society... And of course, their stranglehold on banking and merchants, which made any form of growth under the church almost impossible. Many existing merchants moved to places like France and Britain, or to the colonies to strike it big... Leaving Spain much like an "Imperial housing bubble" ready to pop.
5. And of course, the wars with the Dutch, English, Ottomans, etc. did a lot to deplete coffers and such.