I had an R53
MCS and went to an R56
MCS
I didn't even consider the GP - 2 seats / MPG / insurance
To me, the GP would be a weekend car whereas the R56 would be a "Commute" car - maybe that sums up the R56, it is a little compromised on some of the "feel", compared to the R53 - but that feel would leave my wife barely able to walk after a "spirited" mini-run, compared to wobbly legs following a mini-run in the R56.
I have found that my R56 can give the R53 S Works a run for their money - but then on a mini run I have found that some of the Cooper / One's are no spring chickens either !
In the end though, as everyone says it's down to personal preference, buy it, then enjoy it