The answer is Mark, that I don't think there is an answer. I find that the best selling plating is influenced by the type of pen and the type of sale. It's maybe a question of sucking and seeing, and gaining experience in what sells best where. Its not really a question of what you like, its what your customers like and I find that a shop environment yields different sales to an online environment, which is different to a craft stall environment.
For example, gunmetal slimline stylus pens outsell chrome and gold put together by about 30 to 1 for me. Gold click sierras, which are my best online seller, outsell chrome by about the same margin. Sales of them to America used to be exactly the reverse, but now they seem to prefer gold too. I can't sell gunmetal elegant sierras or zetas online for love nor money, but they are amongst my best sellers in the shops. Gold fountain pens outsell chrome by about 20 to 1 to the point that I rarely buy chrome FP kits anymore.
99.99% of my pens are sold without any influence from me, because they either go online or through a shop, so the finish really is down to customer choice, not any sales patter from me.
I suspect that if we could collectively dream up a posh word for chrome, it might be a different story, but my theory is that chrome smacks of cheap and car bumpers, gunmetal has a mysterious, modern ring to it, whilst gold has connotations of quality, so as far as FPs are concerned I would always expect gold to outsell chrome.
Rhodium is a special case for me because I reserve it for high price/quality kits, so its a bit difficult for me to make a comparison. If I market a rhodium pen, generally speaking they get a choice of rhodium or rhodium - there is no alternative finish to compare it with, and I can't say that rhodium is less popular simply because for obvious reasons I sell fewer high priced pens than the lower priced ones.
It will be interesting to see how my epsilons perform, which I only have in chrome. They're not online yet and I've just started putting them in one or two shops, so we shall see.
It will also be interesting to hear what others have to say, particularly the big market stall sellers like Neil F if he catches on to this thread.