Two topics of interest around that, I find:
1. Sort of a cultural comparison. Fleet of 500+ for WN, around 200 for FR. WN more established, with possibly better all-around service. FR on other hand maybe flying many more leisure travellers, with less service demands? Is it laid-back relaxed Texas culture vs brash Irish/Brit culture. Dallas vs Dublin home? Are they actually quite similar? Do both operate similar hub systems? Award programs? LCC-Competition, harder in US or Europe?
2. What if they were exchanged and serve the opposite side of Atlantic? Would 500+ WN aircraft flood Europe's low-cost market to near extinction? Would FR open markets in US even more aggressive, may be suck in Canada and Mexico? Would US passengers accept the very limited FR service culture, and would Europeans appreciate WN?
I have never flown them, but kind of think I would prefer WN over FR. FR I would compare to a noisy kid. WN much more resolved and calm, may be?