wouldn't the nickel finish and 8 3/8" barrel be more desireable (and, thus, worth more) than the more commonly available blued versions with shorter barrels?
No, not at all, to the majority. The highest demand barrel length is 3.5in for 27s, followed by a 5in. Many shooters like the 6in or 6.5in guns, although they are common. 4in 27s are on the scarce side. 8 3/8 guns are somewhat scarce, but their demand is less. I think with 27s, vs 57s and 29s, the 8 3/8 long barrel was more popular on those bigger calibers. Many people wished to carry 357s back then, and add in that many people feel that a 357 is too small for even medium game, and even if someone uses it, its 50 yds or less as a rule of thumb. Its yet another case of right butt for the seat, because certainly a nickel 8 3/8 27-2 is somewhat a scarce/uncommon bird, ("rare" has a different meaning) but on the other hand, who wants one that long in nickel?
Nickel finish's impact on gun values is a funny thing. Many people prefer blue guns. On the other hand, for those who like nickel, there is usually less to choose from than blue examples. When talking old guns, often times the blue did not hold up as well, and so when talking high condition old guns, some people prefer blue because out of X number of high condition guns, often the majority are nickel. I myself am impartial, whichever is more scarce I probably like, when its an older high condition blue gun, that suits me well too. I have a fair number of both in my collection.