The difference has to do with how the load is supported. In a semifloat, the axle shaft bears the weight along with the axle housing. In full float axle, the load is carried totally by the housing. This is why you can pull and replace a full float axle shaft without removing the wheel or jacking up the vehicle. You can also visually tell the difference as full floats will have a hub at the ends.
To answer your question, full float axle is stronger than the corresponding semifloat axle.
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