Let me try to comment:
1. Historical (1880-1960)
Bronze or brass were traditionally used for UK (and world-wide) naval components and valves. The reason was a relatively cheap(historically) material, cheap mass production (castings), excellennt corrosive resistance (sea water), and the excisting available technical knowledge and production facilities. All based on a broad market for low pressure steam up to higher (railroad and stamengines)
2. Newer developments (1940- and on)
Bronze and copper materials increasing in price (electrical and elecronic uses, worldwide expanding demand), new technological development of stainless steel grades, new technology for mass production of steel and steel products, new corrosive resistance requirements and developments for stainless steels. At the same time steam market worldwide has decleaned drasticallyy. Facilities for copper/brass production has declined, and for steel gone up.
3.'Best material for a specific product'
Within the spectre of applications (for steam components) where both bronze or stainless steel could be used interchangably (specified alloys for the specific use) I doubt you would find the one 'better' with longer life than the other . On the other hand you might find applications (for instance for extreme temperatures and/or pressures) where only steel can be used. On the other hand specific and extreme chemical conditions might require brass/copper (or steel) only.
4. General
For any product a user should go for the product with the best price over liftime (Best = Price/Liftime). A products total cost will vary with material cost, fabrication cost, storage, distribution and selling cost, cost per production lot etc. These costs will also vary with time, location and excisting facilities.
5. Conclusion
No general conclusion. Copper/brass might be cheaper (equal in quality) for larger series of small components. Stainless steel needed for higher pressures. ..But all depending on local conditions, - and dont forget competition!
