Depends what you want from it. I still think the FI is capable of way better sound shaping than the C4, especially now that v3 has full ADSR control (my constant nagging paid off). The C4 has the advantage of a slick interface for auditioning and saving sounds whereas the FI is still a bit clunky in comparison (though I’m hopeful of some improvement in a post-v3 release and have made suggestions which didn’t fall on deaf ears). C4 has a bigger range of filter types but for envelope control, oscillator options and parameter routing possibilities, the FI blows C4 away.
Both have great tracking.
I do enjoy both though but if I absolutely could only choose one, I personally would choose the FI.