There are pros and cons to both WN and the "legacies". I primarily fly alot on UA so I will compare UA to WN.

Seat selection is a wash in reality. Infrequent travelers will get separated middle seats whether they fly WN or UA. Frequent travelers will get prime seats and will board earlier.

No change fees on WN is nice. But for a frequent traveler this is also a wash. Free SDC for UA Golds+ means that I can move to an earlier flight for free if my business meeting wraps up early. For leisure travel as long as I book using points, then all tickets are fully cancellable and refundable on either WN or UA (UA Plat benefit). WN wins here for infrequent flyers.

No checked bag fees on either WN or UA for frequent flyers. WN wins for infrequent flyers who like to pack more than they can carryon. Personally, I rarely check bags regardless, but some leisure travelers are not used to packing light so I can see them benefitting here.

IRROPS are much, much, better if you are traveling on UA simply because WN has no interline agreements and therefore has much less in the way of options. Win for UA for all travelers.

UA has lounges available and if you are a UA elite you should probably get the Club card which gives access as well as 1.5 miles for all purchases. Given the state of UA lounges, however, I can't call this availability a win. They are best used in cases of IRROPS for re-booking, but the IIRROOPS advantage is covered above.

So net, net, IMHO it's a slight win for UA if you are a frequent traveler, and a clear win for WN if you are an infrequent traveler. Others opinions will vary, of course.