Jeff,
One big issue with Spain is their version of the “wealth tax” which is levied on your world-wide assets. It is tiered, but if you do have assets (not just cash, but also real estate, vehicles, art, jewelry, etc., though some might be hard to locate for the authorities), you might end up with a substantial annual tax bill on the property that you have already acquired presumably paying due taxes. A friend of mine who moved to Spain has overlooked this “nugget”, and is now somewhat regretting it. I certainly took that into account when selecting Portugal.
Spain is far more efficient in its GV programme, but beware of hidden costs.