You would be very fortunate to get on at either company. As a FedEx guy I can tell you what I love the most is flying internationally. I have lots of friends at SWA who love flying there and it is a great company. I would say the best thing about FedEx is the ability to choose domestic, widebody, narrowbody, or intl. Depending on your state in life and seniority, sometimes one is a better choice than the other. The other thing is that most of our airplanes are widebody so although SWA has great rates, there is a point where wb rates are just going to be higher. Flying all night and doing hub turns does suck though, no doubt about it.
Had I not seen how much fun the international stuff is, I probably would have gone to Southwest and never looked back. As it turned out for me, just like a lot of guys, I went with the airline that called first. One last thing, do you live in a SWA base and intend on staying there, because that seems to be a pretty good lifestyle.
Good Luck!