My interpretation is that the C4S is more road oriented, whereas the GT3 is more track oriented.
The GT3 comes with sports bucket seats I believe, so no rear seats and access to the rear is more limited.
The GT3 also lacks the centre portion of the console forward of the gear lever, from what I can see in the piccies, so no sat-nav (which may be useful if you want to tour) or PCM.
C4S suspension is 10mm lower, GT3 is 30mm, so it has to be harder.
C4S has traction control, GT3 does not, C4S has 4 wheel drive, GT3 does not. So GT3 will require more concentration in awkward circumstances.
GT3 has aero kit, so is little less discrete but does have narrower rear arches.
From a performance perspective, the GT3 has more power at the top end, above 6500 rpm, so unless you rev it hard there is not much in it (the torque curve is the same). However, the GT3 is 10% lighter, hence the improved performance.
I think it depends on what your primary use of the car will be.
I ordered a C4S 3 months ago and it arrives in 2 weeks. I considered whether I should have ordered a GT3, but I want to use the car everyday. I might be a heretic for sacrificing outright performance, but I wanted the car fully featured.
Rgds, Stuart.