No, it isn't objectively. You are subjectively choosing the criteria that makes one purportedly better than the other, yet they have zero applicability to the use or operation of a smartphone.
Grip matters very much with a smartphone. Rubber breaking down? Has anyone ever actually seen this happen? Is this actually a problem? Torn or stained? Again, is this actually a problem in the context of a smartphone? I would say no, it isn't at all.
A soft, comfortable, lightweight device that you don't have to immediately shroud in a protective case is, by actual use-based criteria, arguably the better "build quality". Build quality is suitability to purpose, not aesthetics or cult of material.
You have different criteria than robustness - understood, grip is important to you. Grip has its own objective measurements.
Christ.
- Rubber breaks down faster than metal
- Rubber stains more than the metals you'd use in a phone
- Rubber can be torn, metal can't
So yes, objectively.
If you drop your phone a lot, rubber might help, but you'd need more than what the Fire phone provides.