It all depends on what type of cruising you want to do.
Formula's forte is upwind/downwind angle.
UltraSonic's forte is like a softened and easy-going/forgiving Formula in rough conditions.
Both are big as compared to an iSonic 127 for a light weight when it gets windy and choppy.
Being 65 kg myself, I've found no difference in low planning threshold (= 7 knots wind + active pumping) between an iSonic 127 and the UltraSonic, both with 55 cm fin + Reflex II 11.0 m2. A Formula is a bit better, i.e., starts planing at 6 knots of wind (70 cm fin + Code Red 2 11 m2 sail).
Whats is your current 10 m2 sail + board + fin combo? Do you have smaller sails and boards if wind blows above 12-14 knots?
How would you rate your pumping skills ? If intermediate, better take an UltraSonic. If advanced, an iSonic 127 might be good enough.