At least with SWR you get larger seats (sometimes more comfortable too) and amenities like wireless charging and curtains. On Southern you only get a nice napkin to rest your head on!
It's ironic then that the seats used in 450 first class are the same as in 377s in both standard class and first class. I've only spent a short time in 450 first class (a slightly naughty test of the seats when coming into Waterloo) so I don't know which variant of the Compin 'Electrostar seat' it is, as they can vary in comfort significantly. The lovely soft variant on the earlier batches of 375s are the best ones by far, but the updated first class variant used in most later batches of 377s is also very nice.
Though the worst has to be the First Class of the 458s - which used the same Grammer seat's as FGW's Standard Class - making Standard Class more worth more than First.
I haven't been on a 458 for a while but I remember my experiences on FGW HSTs very well, and I struggle to believe the IC3000s in first class are nicer than the seats in standard class.
It's also worth noting that various German trains use IC3000s in first class, sometimes with Grammer C3000s in standard, those being the seats used in the Class 395 and despite being one of the lowest tiers of seat in their range I ironically see more complaints aimed at the IC3000 than the C3000 by a significant margin. While both are comfortable, it's the high backs of the IC3000s which caused problems for a lot of people.
Southern first class is a joke. Some of the seats are positioned with their back AGAINST the window for goodness sake.
A design flaw caused by the expanded legroom it seems. I would complain, but in truth I love the inwards-facing seats for their improved privacy.