Hi Scott - Where are you based out of? Here in San Diego we have around a dozen Inter 20's, and also have an I18 and an I17R. I have sailed the I17R and it is nearly identical to the I20 in all respects of handling, quality of workmanship, and user-friendliness. I am sure you would be very happy with it.
I have not sailed either the FX-One or the Fox, however I have seen the Fox. While it has many good qualities, it did not appear to have the same quality of construction or user-friendliness as the I20/I17R. For example, the jib cleats adjustment did not look very solid/subject to slippage, and the rudder setup wasn't as clean. I would highly suggest that you get a test-sail for a couple of hours on the FX-One as well as the I17R before making your decision, so that you can get a good comparison between the two.
There are about 4 Foxes up in L.A., but I believe that's all in SoCal (there are none in San Diego to my knowledge).
Sail fast and have fun,
Alan Thompson
I20 - San Diego