Most experienced riders would feel a difference in the blindfold test.
(thanks Ola, to save the re-typing on this)
As for the size choice, you could very well take the Futura111, especially if you will often be on 7.5m. For 6.5m/7.5m OverDrive (grunty powerful sails) the sweetspot Futura size is definitely 111 rather than 101. 101 is a valid choice, but be aware that the 7.5m OD really will have you at the upper (sail size) sweetspot limit- still sailable but not as sweet as 111.
ST115 vs Futura111 - "Fastest" is always subjective or influenced by too many variables.. Top speed when both fully lit in open water conditions is pretty even but I would say advantage Futura111 due to EASE of speed over a wider range, bottom end (plus general all round nature) and (maybe suprisngly) better sail/power match to OD 7.5m.
Cheers ~ Ian