I think that Southeastern mainline/HS is very good commuter stuff compared to Southern, GWR, c2c, etc (except the 375/9s, which for some unknown reason have 3+2 seating yet don't appear on any metro services*). Personally I like the Electrostars but my favourite is the /8, which is harder but is more supportive and doesn't give me a numb back side. None of the Electrostars, though, are as nice and airy as the Javelins, even with Southeastern's brilliant refurbishment.
*Off-topic, but talking about 375/9s, I think if they removed the third seat, added armrests (the 379s have the same seats but with armests), maybe bigger tables if they can find any, and maybe better baes, they'll be fine for mainline services.