A Weber Premium 57cm would set you back around £170 & a WSM 57cm would be around £380 - so £550 plus a few mods / toys to go with it. That compares to, say, a KJ at £650 (cheapest I've seen) plus toys. So you're talking £150-200 difference I guess.
To be honest I didn't even consider a 'traditional' Weber style kettle or know about WSMs - but had I known then what I know now, thanks mostly to this forum, I would have given the Weber option much stronger thought. I think my view of kettle BBQs has been tainted by the really rubbish imitations we have (probably) all owned at one point or another.
Saying that, I am a very happy ceramic owner & do think that they are more robust - however, I tend to look after my kit & I know there are lots of happy Weber owners who have had them a long long time so not sure that makes much difference. But I do like the substantial (i.e. damn heavy!) feeling of them.
Out of interest, can you get the kind of really high temperatures (700-900F) out of a kettle? That was one of the main reasons I went for a ceramic - to do really good steaks. But have also found that homemade pizzas at that temperature are great too. That was before I fell in love with low and slow....