Upon graduating from a Spanish (or other European) medical school your options will be:
A) Go back to the US, take the USMLE and apply for a Residency.
+ No or little school debt.
- Longer duration (most Europ. med schools are 5-7 yrs, I -think- Spain is 6)
- You'd be an IMG, so it WILL be more difficult to get into competitive residencies.
- You'll probably have to arrange clinical US rotations yourself (which you'd need for Letters of Recommendation).
B) Stay in Spain after med school.
+ Will compete for Residency spots on equal terms with Spanish peers.
+ You'll stay in Spain (presumably, you like the country).
- To get Residency, you'll need work permit. I've heard it's as difficult, if not more so, than getting US work visa.
- Generally, Residencies in Europe are longer (but less intense)
- You'll NOT be able automatically to work in other EU countries, as you're not an EU-citizen.
- Pay after Residency will be considerably lower than in the US.
MOST IMPORTANT:
Note, that these two options are basically mutually exclusive. If you do Residency in Spain, or other place outside North America, you will NOT be able to work as a physician in the US, unless you redo your entire Residency in the US (with a few, very narrow and specific exceptions).
Likewise, if you train in the US, but decide to go back to Europe, it's basically strictly on a case-by-case basis if they'll recognize your US training. Some get it fairly easily, others must redo everything, like in the US.
Generally, I think that people should think really, really carefully, before entering a medical school in a different country from the one they intend to work in.