I went through this decision, initially purchased steel flares, returned them and ended up with fiberglass.
The fiberglass is more 'correct' from the viewpoint factory GR4 used FG, also the FG has a better curvature than most steel ones I've seen and gives a better final look. FG is easy to fit and easy to repair if it comes to it.
If you are in Vancouver, consider going down the road to Abbottsford and Kodiak Motorsports. I have Kodiak's on the GT5 and another spare new set ready for the GRP4. A full set can still be had for around $3K.
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