Marseille to Barcelona is 4h30 by train, so slightly longer (but arguably much more civilised) than an end-to-end air journey (which would be about 4h, when you account for travel to the airport, check-in time, etc, etc). However, there is only one direct journey per day and it's very popular. Do book ahead of time on http://www.voyages-sncf.com/ to get a decent price, and to guarantee a seat - the trains are reservation-only.

From Barcelona to Seville, there are some direct AVE trains, taking around 5h30 (which can be booked on http://www.renfe.es/). That's definitely longer than a flight, although not excessively so.

Direct flight are operated by Vueling and Ryanair. I'd rate the former above the latter. Both are LCCs, and reasonably serviceable, as long as you bear in mind the luggage fees and stay strictly within weight and size allowances, ensure you check in online, etc, etc. I wouldn't bother with a flight via Madrid just to fly on Iberia - not worth it for a flight of this length IMO.