For what you want to do the Isonic 50 is by far the best choice. (I have ridden both a lot)
1. You have already sailed it and know it feels good for your weight and done a PB in it.
2. It has a slightly wider tail and area under the feet which will be better at the speeds you will sail at and it is far easier to get going then the very narrow, pin-tailed F2
3. The amount of litres means virtually NOTHING in this size of board for the conditions you will sail it in. It is the shape (width, outline and rocker) that define it's characteristics. Do not even consider litres!
4. Remember that I set my best ONE HOUR speed of 28.8kts on the Isonic 50. Does that tell you something about it's ability to keep going though gybes and lulls?
5.The IS50 rides flatter in the sort of small chop you will likely encounter speed sailing, and therefore with less drama. It may be ultimately slightly slower in perfect flat water, but it will be faster for you in real world conditions because it will give you the confidence to push. (I still got 44.6kts out of mine)
6. Don't even consider getting anything bigger or wider than the IS50! (eg. Manta) I repeat: FORGET IT! It will be a waste of time and money. You buy a speed board for one purpose only. To go as fast as possible in the best conditions you get. For your size, anything bigger (and by that I mean wider -
not litres) is a slower slalomy type board. You already have that.
7. Lena, who is about your size, was using an approx, 35cm wide board at Luderitz when I was using the CA40. Smaller people need smaller boards to be fast, even at 35 -38 knots. When you are ready for Luderitz and low 40's+ you will need an even smaller board, but until then, the IS50 is it!
8. I will send you my spare set of footstep setback plates to set it up for your stance.
9. Get it now and do it while you are still young!