Which iPhone 5 model sold in the US should I buy for use in Spain?
Verizon ships its iPhone 5 unlocked, and it operates on the LTE bands that are typically used in Europe. The "GSM" model sold in North America is restricted to LTE bands that are currently only used in Canada and the US. Both models can operate fine on 3G GSM networks.

As far as I can tell, while there was an LTE spectrum auction in Spain last year, the precise bands to be used haven't been published yet, but the Verizon (CDMA) seems to be more useful currently, and may be more likely to work on future LTE networks in Spain (but this is far from guaranteed, unless someone can find the exact bands to be used).