So I now have the C4 and the SY-1.
First impressions are that the C4 takes up less space than the FI (in its original case) but I'll have just as steep a learning curve with the software. FI wins with regard to having proper midi in/out, C4 wins with simplicity of 'on the fly' controls to change essential parameters live. FI wins with the ability to scroll through the patches with a footswitch as opposed to the C4 requiring fiddly finger fiddling.
SY-1 beats them both for straight out of the box simplicity. Plug in and move through a host of presets with ultra responsive simple controls to dial the sound you want.
Tracking, and sound quality is great on all three, but don't be put off if you've used the old Boss synth pedals, this one is the real deal. Every single sound is entirely customisable and perfectly usable.
The Boss is straight onto the board and ready to be gigged, the C4 has to be on and off for programming and almost certainly won't be 'right' when used with a band (it's never the same in the mix) however unlike the FI I will be able to tweak the basic parameters.
Time will tell, but the SY-1 is winning right now.