My MKI stock experiences were mainly on the GEML on:
all of the non-corridor slam-door stock, (those 6-a-side bench seats were clearly necessary with the Gresley-style single bolster bogies) and heavily used jointed track
the 306s which were quite a bouncy ride and the suspension sometimes seemed to bottom
the 309s were fine most of the time but by the mid-late seventies, the Commonwealth bogies definitely had a rougher ride at speeds over 70mph.
My SWML travels were in the late '60s mainly:
in 4-SUBs as far as Wimbledon, once again the bench seats were terrible and on 4-CORs from Waterloo to Havant. That was a lively ride where bouncy seats were really needed soujth of Guildford.
In the late '70s and '80s it was mainly thumpers from Portsmouth to Fareham, when 60mph was the maximum preactical speed and CIGs/VECs up to Waterloo. All of theose plush looking seats were quite tiring with little lateral support and as I said above, the padding was definitely unfit for purpose for most of its life on the most used stock
The only breath of fresh air was the 442s when they were new. The seating was refreshing change but I moved away in the early nineties so only had a short experience of them.
The MML is not an easy line for suspensions, especially the fasts between West Hampstead and The Elstree Tunnel, but the better support in the Electrostars, (especially the 387s) makes it bearable despite the uneasy ride of those trains. The 700s however seem to eat the bumps and the seats are quite compatible with both comfort and (hopefully) durability. Far better than the soggy pseudo-luxury of the 377/2s.