I noticed the exact same thing when I test drove the Ed30 I was thinking of buying. The interior was definately more modern than the Mk4 but the plastics seemed to be a bit cheaper. It's a similar story in the R32 to be honest. I wouldn't say the Mk5 looks cheap or badly made, but it doesn't look like the plastics are as good as the Mk4.
The more modern look of the Mk5 (to me anyway) offsets that negative.
There isn't a massive difference between the GTI and R32 really, the R32 has some aluminium bits here and there and nicer seats but that's about it. Does the GTI have a dark coloured roof lining?
The stock audio in the Mk5 isn't a patch on the Mk4 though, infact it's pretty poor. I wasn't overly bothered as I've been abusing that V6 and making music of my own but a couple of days ago I fitted the new touchscreen hard disc sat nav and the sound quality is much, much better even with the standard speakers.
I think that's where it's going actually. I was in a car being rushed around the Brussels ring road when I made the arrangements so it's all a bit hazy!
The car is going in for repair a week today and should be gone for 3-4 days. In the meantime I don't know what I'll be driving, but I've been told it'll be at least 2.5 litres and probably a straight 6. So a Beemer of some description I assume.