I disagree strongly that the service is much better on BA. The flavor of service is very very different but I find the service on LA to be wonderful. That plus the catering is MUCH better than BA. I had some of the best seabass I've ever put in my mouth on SCL-MAD. They also haven't cut their caviar service, and the A340 has a vastly superior IFE system with truly great quality audio/video which is fully on demand. They don't hand out pajamas but are very willing to store your clothing if you bring your own. As for the seats, I find the 2x1x2 config to be passable, but the BA suite is indeed superior. The seat is perfectly comfortable however, and indeed I slept 8 hours. I've never connected at EZE but connecting at MAD is very very simple and the IB F lounge is rather nice (but NOTHING like BA at T4). I did SCL-MAD-LHR earlier this year and it was a snap. You'll come into terminal B and IIRC the LA flight boards from the same terminal so you won't even need to transfer. If you want more mileage and don't mind the extra time, you can do LHR-FRA-SCL which has a stop in MAD before continuing on the same aircraft to SCL.

FWIW the seats are identical to the seats AA uses in F on the 763s. If you've done other BA segments in F and have experienced their F service, why not try something new?