I would go with IB any day, simply because connecting at MAD will be so, so, so much easier than connecting at MIA. The new-ish T4S is great! And the fact that you avoid the pain in the arse of US customs and immigration not just once, but twice (once on the way out, and again on the way back), makes it a winning proposition, at least in my book.

The seats on IB are fine - they are angled-flat and not flat-bed, but they are comfortable enough to sleep quite well. The service is a bit... brusque let's say, but not terrible. Definitely better than what you could hope to expect on AA for example.

Also, I find that the service improves if you speak Spanish, so that may be something to take into consideration.