Just to throw my $0.02 in the ring here - I think the talk of relative pricing, while interesting, isn't entirely relevant to the question. Having had my 4200 for a couple of years, and then driven a GT, I found them to be hugely different cars. The 4200/GS is a much smaller car, a much more involving drive, and (for me at least) hugely more fun. It isn't as relaxing on the motorway, it's isn't as smooth as the GT either suspension or gearbox-wise (I drove a 4.7 GTS Auto), and it's not as well insulated from road/wind noise.. In fact having finished driving the GTS I came away thinking it's much more like a prettier and faster version of my 4.2 Audi A6 than a more modern version of the 4200/GS which is what I was actually looking for. It's more of a sporty QP than it is a GS replacement. So if you think of them as the difference between a 3 and 5 series BMW perhaps, I think that's more accurate than imagining a 3 series and a next generation 3 series.
So, in my opinion at least, I think you need to look at what you're after in a car first, and then work out if there is a Maz which fits the bill. And if there isn't one which matches entirely, go buy either one - they're both great and you'll not have any regrets!
Just my thoughts having driven both. My only caveat would be that I've never driven the 4.7 MC Shift which might well be a bit closer to being the spiritual successor to the GS - though of course it'll still be bigger and more isolated from the road.
Cheers,
Ben.
PS Don't count the 4200 out. If you're thinking a GS at £30k, you could also consider a really good '05 4200 at £15k or so.