Depends on heaps of factors, age, if you are racing, do you want to get planing earlier, do you want to sail when it gets windier. The 127/9.6 will be easier to use and have a bit more top end. The formula will get planing in less wind and will be a little bit more bottom end.
2014 Starboard carbon Speed Special 49
2014 Starboard carbon Isonic 90 and 110
2014 Severne sails OverDrive R5
5.1 5.6 6.2 7.0 7.8 8.6