In my very short experience - as yet - with the C4 I think may actually track a fraction better than the FI with the current firmware. It is splitting hairs, but I was doing some AB-ing of sounds (as you do :)..) and I felt it was just a fraction better.
But I have not tried it with different basses, a million patches etc. so Herr Quatschmacher is probably the better judge.
Comparing the two is very difficult as they clearly are trying to achieve different things. The FI is intended as a full fat synth that is invinitely tweakable, the C4 I feel has been developed to be easy to use and excels in a smaller ‘bandwidth’ for lack of a better term. To me it is more like a Micro Korg, just totally excellent in many ways but not in as may ways as the FI is (which has ADSR, better sustain, more oscillators, separate LFOs for every oscillator which I do not think the C4 has.... not sure though, need to spend more time with it).
So fun, ease of use, size all very nice on the C4, FI the MacDaddy of bass synth pedals. But you do not always want or need the MacDaddy.
Hope that makes sense, and I want to reiterate that I am only just getting into the C4.