Hi Gareth, I find the merc seats very comfortable. But like I say you'd struggle fitting 5 people in the a class with the Amg sports seats fitted. My dad has a e class and I too find that comfortable especially as we do a 8 hour trip to Northern Ireland each year.
In comparrison I found the a 3 seats less comfortable and did have some back ache on a 2 and half/3 hour journey from Leicester to Luton airport. But to be fair I do suffer from back issues (after causing damage to it).
Cheers
Steve