For the chrono pieces, I think a white metal looks most
appropriate. Since you only gave 2 choices, that leaves only
titanium. If you were not restricted to those 2 choices, I think
titanium would still the best choice, simply because that watch has an
especially large case, and it would be quite heavy in white gold or
platinum. The only RMs that I think a white gold or
platinum case makes sense are the ones made by Renaud & Papi.
Eric von Schonberg