If you are sailing by your own and able to manage sails around 10/11 m I would go Formula.
If you are intending to race and sail with other people on RS:X, then I would recommend the RS:X. The RS:X is a pretty boring heavy board by itself but its main point is to be a One Design class and thus to race with the same equipment (which is a lot of fun even in small group).
You can also buy an RS:X gear (9.5) and use it on a formula board if you are light enough to plane in 8/10 knts with that. Then you can either buy a RS:X board if you want to race the Olympic class or keep the 9.5 as your small formula rig for windy days when upgrading to a bigger sail.
under 17 - what about BIC 293 OD ? especially if you "play" on a lake in 8-10 knots
racing = RSX or 293, but practice in bigger winds than 8-10
non-racing = Formula, FreeFormula or simply something that will plane your weight in light winds some people seem to like the new freemoves for early planing, etc
Thanks! I compete in 293, but because I weight 70kg and im 1.80 m tall, it's completly inefficient a sail that size with that wind. I would like to race, but don't know if RSX is worth it
I think it depends a bit on what sort of sailing goes on in your local area. If there is a RSX fleet then best to raced with them, but if it is pure pleasure you are after then maybe Formula is a better option? I should state that I'm a bit bias, as I really enjoy FW....
Using an RSX rig on a Formula Board is not a bad option also.
The conditions where you live is a consideration...