A sweeping generalisation. Before privatisation commuter stock was relatively basic and uncomfortable, while "Inter City" stock was much nicer, roomier, more comfy seats, air-con etc
Since the late 90s, the categories of trains have blurred with a similar standard of trains operating both routes. As a result, trains like the Electrostars, Turbostars and Desiros are far nicer than the Mk3 based EMUs and DMUs, with much better seating, aircon etc, whereas the Pendolinos and Voyagers seems inferior inside to the Mk3 IC carriages, being cramped and in the case of the former, with tiny windows.
The 350s, for example may be slower than the Pendolinos, but are in many ways much nice to travel in.
With the desperate need for extra capacity around London, it's interesting that commuter trains are now becoming less comfortable again, so that more people can be crammed on board, hence the 700s are less comfortable than the Electrostars, but nicer than the 319s...
Although commuter stock built before the late 1970's tended to be quite luxurious in my experience, so I suppose it depends how far back you go.