If your extended time in Paris doesn't allow time in the south of France, using a budget flight will be the quickest and most efficient way to get to Barcelona. I haven't flown with Transavia, but have with easyJet and Vueling and both of those have been great (easyjet is my preferred budget airline in Europe). easyJet has flights from CDG and Vueling from ORY. Both airports are relatively easy to reach from central Paris (I often use the RER).
" We are a bit leary of the low cost airlines, with fears that the cheap fare will escalate with fees and penalities"
It's not surprising that you've heard bad reports on RyanAir (although they're trying to clean up their image lately). Regarding fares escalating, many of the budget airlines use a sliding scale, with prices increasing as the flight fills. Therefore if you book early, the fares shouldn't escalate. However, there will be a few extra fees (ie: checked luggage). Another point to remember with easyJet is that they usually only allow ONE carry-on item per passenger, of the approved size (weight doesn't matter as long as you can lift it). As long as you know the rules up-front, it shouldn't be a problem.
One other comment about easyJet is that they no longer offer check-in at the airport. Passengers check-in and print their boarding passes up to 30 days prior to a flight, and then just use a bag drop for checked luggage.
If you dislike the usual airport hassles as much as I do, there's an alternative. There are few direct high speed trains between Paris and Barcelona each day, with a travel time of ~6H:15M. I doubt if air travel will be much (if any) faster. For example, if you departed Gare de Lyon at 07:15, you'd arrive at Barcelona Sants at 13:30. That would provide time for a brief walkabout after checking into your hotel, and then a nice dinner. It would most definitely be a less stressful and more relaxing trip than by air. Have a nap or a nice lunch in the dining car, while you watch the scenery flash by at 300 km/h.