I'm reluctant to give same-day-as-arrival scheduling advice because so much can go wrong when airports are involved. There could be a delay in Immigration as well as a delay on the flight itself. Then you have to get into town, to the Atocha Station, which means waiting time before your train/bus departs the airport and then the transit time into town.
Checking Renfe, I see that there are hourly trains to Sevilla-Santa Justa, plus the occasional extra train. The May 2 tickets cannot be purchased yet, so I looked at Tuesday, April 18. Fares for the late-morning/early-afternoon trains are currently between 47 euros and 60 euros; perhaps they were a bit lower when they initially went on sale. Compare that to the 75.50 euro fare I see for tomorrow's fast trains (it's too late in Spain to see much available for today, so no decent comparison; you could check when you first wake up tomorrow, though).
So it appears that waiting until the day of your arrival might cost you just 20-30 euros extra. Perhaps there's someone here who has bought a same-day ticket to Sevilla and remembers what he or she paid. To me, buying in advance is at least that risky. If you pad the schedule a lot, the risk is that everything will go swimmingly on your flight and at Barajas, and you will get to spend a lot of quality time inside the Madrid-Atocha rail station, waiting for your pre-purchased train. Atocha is actually a pleasant place (if you're not in a hurry, it's a bit confusing), but after a night on an airplane, you probably won't care to spend extra time hanging out there. If you are too optimistic in selecting your train time, well, it's obvious: you may have to throw that ticket away and buy a full-fare replacement anyway.
If you opt to buy your Sevilla ticket after arrival, I think you can do that at the airport rail station, which is much to be preferred since Atocha's ticket lines can be more than frustrating.
