beatback I don't know, because I went to a grammar school and also teach in one, so to be honest I don't have the breadth of experience to take a position on which is better. However, my DH was educated at a comprehensive and did very well (and his teachers seems to have taken much more personal interest in him than mine did in me -- possibly he stood out more).
There were a couple of documentaries on BBC 4 a while back called 'The Secret Life of the Grammar School' (or something like that) and I found them really interesting as I hadn't known about the political background to the destruction of the national grammar system in the 60s and 70s.
I would say though that we would consider it quite mad if music or sport (for instance) weren't 'selective'. I do think there is a 'high performing team' effect when you get a group of bright kids together, but that could equally well be in a comprehensive as in a grammar. It is also more likely that some subjects are going to be offered/taken up in highly academic settings. For instance, a large proportion of the small number of girls taking Physics A level are in grammars and independents.
On other hand 10 years old is very young for such an important test and the stress around 11+ now is unreal (I think the galloping fees at independents have something to do with that as well, however).
What do you think of streaming -- is it better to stream within a school than to separate children into different schools?