Wanted to offer a couple other observations, more as "food for thought" for those still reading & learning about the dynamics of airline pay.
I'll agree with Sandman that it is highly possible for a FedEx pilot who works his system to make a LOT more than an average SWA pilot, while flying significantly fewer hours. It is ALSO possible, though, for a SWA pilot in the right position to also make a lot more than the average SWA pilot, quite possibly while also flying fewer hours. Each airline has its tricks which work if you're situated & inclined to take advantage of them... sit reserve at home in domicile, pass (or if possible give away) assignments, volunteer for extra flying any day any time if the premium price is right, etc. Good on the guys who can make it work; my life isn't flexible enough to drop everything on zero notice to go fly some outrageously high paying turn, but some guys live for those calls. Scheduling loves them, and their willingness to fly that stuff means fewer JA ("sorry, Snoopy, but we need ...") calls for me!
There are people at SWA as well as FedEx who've done very, very well, beyond all averages. The captains who fly widebody jets for 15 years have a very good thing going, particularly if $ is the big thing you look at. Not everybody will be a widebody captain at FedEx for 15 years, though. Just like not everybody at SWA will be a captain for 32 of their 34 years at the airline with all their original profitsharing in SWA stock since the mid 70's!
When it's all said & done, a widebody full of priority overnight small packages is carrying a LOT more $ of revenue than a 737 with (max) 137 people who paid (at most) $299 for their ticket... so even flying several legs in the 737 that day, and much of the revenue from the packages going to pay for trucks & drivers & sort facilities & etc, the freight guy at the high end probably can do better in years 32, 33, and 34 than his twin brother at SWA. So, if you're 26 and have offers from SWA and FedEx and total earnings is your sole criteria, I don't know that I have much argument to tell you that you'll do better at SWA than FedEx.
For a shorter career, though, reference the numbers Falconjet posted above, and my earlier comments about uncertainties & the difference being well within the margin of error for forecasting your future careers at each place. NEITHER job will leave you in sight of the poorhouse, unless your money all goes to multiple ex-wives & insane mortgage payments, in which case neither job will pay you nearly enough. They're both good, and I'll go back to what I & others have said before: be aware of the (best guesstimate) financial side, but consider the lifestyle factors: day flying vs years on the backside of the clock, dealing with pax & F/A's, or flying boxes, living in a domicile vs commuting for a career, flying big jets across lots of time zones vs domestic layovers, etc etc etc. And be cognizant of things like seniority -- bird in hand vs bird in the bush, 2 year headstart at FedEx, etc.
For *ME*, the difference between a (totally hypothetical numbers here) $4M career and a $5M career is not NEARLY worth the stress of working backside of the clock, then swapping around to have a "normal" day with the family, then swapping again next time I go to work, FOR THE REST OF MY CAREER! Not even close! But that is only just me, and for plenty of people that lifestyle is no problem at all, and I'm glad that it works for them. But there are reasons SWA was my first choice & FedEx wasn't, just like many people wanted FedEx above everything else. Doesn't make anybody stupid, or ill-informed, or wrong, or mistaken. Just a reflection that people's priorities differ.
My recommendation is to consider YOUR priorities, all of them, and don't let some financial estimate of the next 20 years be your only guiding factor.
Sorry for the long ramble, but maybe a few people may get something worth considering out of it.