Hi Sarah,

This will be the second year that I sail the FX-One and can tell you that its a great boat which is easy to sail and gives excellent performance. Personally I think that the Inter17 is slightly slower and heavier that the FX. Solo righting will not be easy in light winds, although I weight 75kg and have'nt had a problem yet. (If in doubt, order the righting pole. problem solved). I have yet to sail an F16 so cannot really comment on that. Although I saw a Swell Shadow last year and thought it looked really nice.
http://www.swell-catamarans.co.uk/
I sailed alongside one (downwind) during the Round Texel race last year and was under the impression that it was slightly slower than the FX. Its spinakker is 10m2 compared to 17m2 on the FX.

If there are some FX-One's at your club already than that would be even more reason to get one.

Have fun! :-)
(Wouter, will you be attending the Texel Dutch Open?)