From the knowledge I have, the problem between these steel and keycaps is very different, mainly because of the conditions where the material is used. Choosing a material mostly depends on the characteristics that you search for, and the price. For these metals, they could choose thickness from what you said, but weight can then be a problem.
For keycaps, flexibility is not supposed to be a need since the keycaps are not exposed to stretching or hits in normal circumstances. Usually, broken keycaps are broken during transportation because they were put under lot of weight or hit hard.
When you type, you don't hit them hard or stretch them. So then there's durability. Basically, both plastics are durable enough for the job, but abs will shine faster. So if this bother you, get pbt, if it does not, get any material you want. There will be a little difference to the feel on your fingers also. At this point, it's all about personal preference, but both can do a good job.
