This depends on your definition of "strong." If you mean "has a harder surface and is more capable of withstanding abrasion," the porcelain wins hands down. If you're subjecting a part to heavy but gradual compression loading, again, porcelain wins. BUT, if you're talking about putting a part into bending or hitting it - impact loading it - fiber glass will win the day.
Fiberglass deforms and has a high elastic limit compared to ceramics, which are very hard but also very brittle. When a ceramic fails, it fails catastrophically - breaks through completely and therefore loses all strength. Fiber glass will bend ... and bend ... and finally crack, but not necessarily break completely.
These differences are why both materials continue to have applications.
Fiberglass deforms and has a high elastic limit compared to ceramics, which are very hard but also very brittle. When a ceramic fails, it fails catastrophically - breaks through completely and therefore loses all strength. Fiber glass will bend ... and bend ... and finally crack, but not necessarily break completely.
These differences are why both materials continue to have applications.
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Fiber Glass.