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
2016 Starboard Isonic 80, 97,124
2015 Severne sails Reflex 6
5.6, 6.2, 7.0, 7.8 and 9.0.