My background, so you know where I'm coming from, is a year and a half at Flexjet, then 2 years flying the 727 freight, then furloughed and into a RJ, then finally on with a Major, so I have a "somewhat" unique perspective of having done it all. I'm sure there's others who have as well, and hopefully they will also post their thoughts.
1. If you were at Netjets, I'd tell you to stay put. If you are anywhere else (which you've stated you are), either UPS or SWA would be a good bet.
Flexjet (and the other operators) don't make their PRIMARY cash off operations, they make their money off aircraft sales. During the VERY FIRST pilot meeting management called in 1998 at Flexjet, they came right out and said that, as well as stating their primary goal for Flexjet's operational wing was to simply break even. Period.
Anything that affects the aircraft sales market affects whether they stay in business, and that goes for just about everyone, Netjets simply is less affected by sales fluctuation because their operational wing actually DOES make money, so the rest of the fractional operators are only as safe as the economy.
2. Like others have said, get the job offer FIRST, then stress about which one to take. Say "Yes" to all of them, then sit back and think about it before turning in your notice.
3. SWA or UPS? I'd take UPS, but then again, I prefer freight over passenger flying, and I know I'm probably in the minority here. After leaving the cargo side with 16 days off a month and flying pax the last 6 years, I can honestly say the cargo side was a LOT more peaceful with far fewer disruptions that having passengers, not to mention flight attendants, in the back brings with it.
Both FedEx and UPS have excellent long-term viability, they haven't had a yearly loss in I can't recall when, and they haven't furloughed anyone in recent memory, even with the 9/11 downturn.
Certainly a terrorist act can throw all of that out the window, especially if it occurs on a cargo aircraft (I still wish they'd have to scan all the packages just like they do baggage, I believe explosive devices are our biggest security problem). But you can't plan on that or you'd get out of aviation all together.
I hear that UPS pilots are unhappy, but I've met quite a few jumpseating around and they were happy enough, as well as having flown a couple years doing UPS relief freight in and out of the SDF hub a couple times a week during the winters, didn't notice anything especially different from the pilots in our ops area... I think that's all in your perspective. JMHO.
So, in short, my vote is UPS 1st, SWA 2nd, non-NJA fractional last in your list of options. But, like I said, get the job, then figure out what is best for you. Your wife (and future kids) may appreciate you having SWA pass privileges more than a night freight circadian rhythm,,, so maybe SWA is more for you?
Again, all about perspective.