As others have said, if you want the noise of the GranTurismo S w. MC-Shift transmission, then it's the GranCabrio Sport that you want as they have the same exhaust.
N.B. Although the standard GranCabrio has the 4.7l engine that's in the GT S, it has the same suspension set-up and the standard Auto software that's in the standard (4.2l) GT, i.e. not the much-improved suspension and MC AutoShift s/w that's in the GT Sport and GC Sport. Oh, and the GC Sport also has an extra 10 HP over standard GC. Hope that helps.
With regards to GranCabrios at Dick Lovett, they seem to have 2 in atm - a ('Rosso Trionfale' - red) pre-facelift GC Sport @ £65k (
link) and a ('Grigio Granito' - dark metallic grey) post facelift GC Sport @ £105k (
link). It's obviously a matter of personal preference, but out of the two, I personally think the red one looks way better (despite it being pre-facelift), and it's £40k cheaper too! Bonus