I may have time to post details later, but generally legacy programs win big if you can reach their top elite levels. Exception: short-haul flyers like me do better with Rapid Rewards. RR 2.0 will soon kill that advantage. Then I'll have to be content with on-time flights and no change fees.
+1. Fully agree with this. It is fairly well known and agreed upon in this forum that the perks for
TOP-tier status on legacies (ie, UAL 1K) is better than top-tier on Southwest. Possibly the same for mid-tier (UAL 1P), or closer to a wash. Low tier (UAL 2P) I'd say is a wash or SWA possibly has the advantage. No-tier fliers (UAL base members) are MUCH better off on SWA. I think that is the general consensus on this forum after reading it for several years.
However I also think the F upgrades on legacies are highly overrated. Back when I was a PremEx on UAL, I bought a soon-to-expire SWU on eBay for $67. I really debated whether or not it was worth the $67 since I already got lounge access on my Int'l flight. When I became 1K with UAL, I traded all my SWU's away for around $500 value each. So somebody obviously values F upgrades highly, but it isn't me, that's for sure. I'm quite happy in my Exit Row seat with tons of legroom.
So basically, it completely depends on the individual flier and his/her unique circumstances as to what the value of the different statuses are for each airline. But the general consensus is that if you fly enough to earn top tier status on a legacy (especially if any of that flying is Int'l), then you're better off with the legacies. Otherwise, you're generally better off with SWA.