I had a 2008 LS3 Coupe before the GS. Both were UK spec Z51. I don't think there was much difference, and I certainly didn't notice any. However, I could drive the GS at 70mph with the roof down and windows open, but the wind noise at 70mph in the coupe with the roof off was unbearable. Go figure.......
The Z06 is probably more desirable for the perfomance and those that yearn for the kudos of a Z06 badge, but I would have the GS every time. Just couldn't get on with the colour combo on mine. Red exterior with red interior which didn't match. Too much red and not a scrap of yellow in sight.
Goodness knows where the other Man Utd ones went but there was a silver coupe which, at the time I bought mine, was to be avoided. Caveat Emptor etc. but I think you're old enough, Graham, not to have me lecture you.