I notice a lot of the argument these days is less about the hardness of the seats as their height. There are very few "springy" seats on the network at all these days - it's far more about the contours of the seat than anything - a quick feel will reveal that even the most seemingly plush seats have very little material on them.
I'm still in the IC70 camp, mostly because I prefer a low seat back without resorting to a bus seat a la a pacer, and the armrests don't bother me too much. But I'm warming to IC3000 Grammers too - I've grown to appreciate how they cocoon you, though their height makes getting a decent window seat a gamble.
Shoutout to whatever seat is currently in EMT's 156s, as well. I recently took a journey on one in which the engine on my coach was out for most of the journey, and without the rumble of the engine I grew to appreciate that, actually, they're very comfortable and well-proportioned trains when well maintained, as this one was (internally at least!) It was a real shame when the driver managed to get our coach's engine going for the final stretch!
I'd still way rather sink into a Mk. 1 compartment if I had the choice though.
Finally, here's a radical idea - adjustable headrests on trains!
(Though I suspect it would be a vandal's dream...)