I haven't been able to fault the S80 V8's seats.
When Ms Wife & I had to drive off The Barge and straight to Sydney in one stretch, I could still get out and walk around without aches and pains.
Haven't really had long sessions in the C30's seats, but they've felt quite comfy in the 2-3 hour hops I've done so far.
Of the OEM seats, IMHO however the 940/960 seats are a lot more comfortable than 740/760 leather seats.
140 cloth seats are comfortable, but don't have a lot of side support.
Ditto 240 cloth seats, but it's surprising how you can sit in 'em for hours and not ache later.