Because you already have a futura 122, the choice should be fairly easy, i.e. go for the Ultrasonic.
And if you really want to plane in 7-10 knots, then the US is the only logical choice (except formula).
I don't really understand why you bother for 15-18 knots, because these are excellent futura122 conditions. On the other hand if you really want to use the US in 15-18 knots, it's possible. I've already done it with my 8.8 rig. But I prefer to be on my IS122 + 8.8 in these conditions. Even my IS122 +7.8 would get me planing without too much reserve of course. I'm 87 kg.
Like you said, a F131 is much too close to your F122 and although you're a bit lighter than me, I doubt if it would get you planing in 7-10 knots. You'd have for sure a lot of problems going upwind.
You will also need a powerful soft fin (>=62) on the US in 7-10 knots and a 10+ rig. You can use all the lift you can get in these conditions.
IS87 / IS101 / IS122 / US147
Last edited by BelSkorpio; 26th February 2012 at 01:02 PM.