The M6 Coupe (F13) and XKR-S both declare 0-60 of 4.2 seconds and 0-124mph @ 12.6 seconds with the M6 limited to 155mph unlike the RS. The XKR-S in my book is therefore
faster.
The Convertible comparison changes that aswell as the Jag picks up 47kg for a soft top whilst the F12 BMW picks up over 200kg and stunts the 0-124mph to 13.1! So the convertible is faster from the Jag stable. This is a bit of a weight hit to the Coupe Jag as the ally body of the Jaguar was designed for a convertible.
In terms of price you are looking at £94k compared to £97k. Just taking the BMW from the standard 19" to 20" to match the Jaguar would cancel the difference and proportionally to price the difference is negligable.
CO2. Not really a point that would influence decision on this type of car. Its about emotional appeal and that's subjective. Running costs are primarily depreciation.
The Audi V10 is £10k more and neither V10 or V8 are more efficient.... only on OCUK would that matter though
Its not bad when you compare the number of cars from the marques. Audi did what 1.5M cars last year? Jaguar less than 55,000