There isn’t a universal winner. It depends on the lens, the teleconverter or focal reducer, and the specific sensors.

In the simplest theoretical case—same lens, same pixel count, and a crop factor that matches the teleconverter magnification—APS-C crop and full-frame + teleconverter are trying to capture the same central part of the image. The difference is that the teleconverter adds extra optics, which can reduce sharpness, add aberrations, and cost light. So all else equal, the teleconverter is usually at a disadvantage versus simply using the smaller sensor area.

However, a high-quality teleconverter matched to a lens can outperform simple cropping if the lens has enough resolving power and the light loss is acceptable. A poor converter can be worse than cropping.

The same caution applies to focal reducers: adding extra glass can help or hurt depending on the exact combination, and third-party adapters vary.

Bottom line: there is no general rule that full frame + 1.4x is always better than APS-C, or vice versa. For image quality, the result is only as good as the weakest optical link, so real-world testing of the exact setup matters most.