AVE train or plane from Madrid to Barcelona?

Even a good on-time record doesn't guarantee that your particular flight will be on time. That would be an unprotected connection and you need to allow lots of time between the flights. Personally Inwould go to Madrid the night before.

I would NOT plan to travel from Madrid to Barcelona on the same day as your flight back to the States. Too many things can go wrong and then you risk missing your flight home and that will probably result in additional significant cost to you. Go the day before. My experience with Vueling in Spain resulted in a several hour delay. My experience with Iberia leaving from Madrid resulted in an entire day delay, flight cancelled. Personally I would travel by Ave train to Barcelona. I would give myself that extra cushion of time and peace of mind by going the day before.

I would bet btw travel time to the airport, time at the airport to check in and get through security, that it would be faster to take the train. We took the AVE train a few years ago from Barcelona to Madrid and it was easy and painless. Definitely go the night before, though. Don't take a chance.

The train may be more expensive, but at 2.5 hours between city centers and with frequent departures, it's our preference. Locking in a particular departure two months ahead gets you access to the better discounted rates. I would not choose the last one of the day.

Vueling is a reliable budget carrier owned by Iberia/British Airways. But like others have said you may want to go the Madrid the night before.

I recently flew Vueling from Granada to Barcelona with no issues, however as others said I would be hesitant to schedule the flight for the same day I am departing in case there are issues.

For Madrid to Barcelona I would be inclined to take the train rather than flying (probably more reliable, and just as quick or faster once you factor in travelling to/from the airport and airport lines), but would still book for the day before rather than the day of the flight.