I would absolutely go for a board where you can use a 9,5 sail. With sails up to 9,5m you can manage with a relatively short boom.
The US is probably easier to sail than a Formula.
The IS127 will not plane as early as the US, but as OZI says it will give you more fun when it gets windier. The IS127 works fantastic with a 9,5 sail, and it also works perfectly fine with a 7,8 sail with 46cm fin and steady winds up to 12 m/s.