GMT !
As said above the law only requires the flight time BUT having seen peoples log books investigated by the CAA during CPL/ATPL issue I think that it is a distinct advantage to have the start & finish times recorded.
It makes proving that you have done the flying very much simpler as the times in your logbook will tie up with the airfield logs that the CAA will look at to check that the flight was made.
Using GMT just keeps the whole thing standard and simple as this is what should be on the navigation log.