When we rode the White Pass some nine years ago I asked the conductor of our train which of the two types of diesels were "better."
From what this individual told me the 90's "turned easier" and as a result weren't as hard on the track or prone to derailing.
The Alco's, called 101's by the White Pass crew at the time, had to have work done to get them to "turn."
I have no clue what that means.
But if you're going to pay for one of these locomotives you'd probably want one that was easier on the track, thus a 90 class.
Who knows, but these units were made for mountain railroading and in all likelihood would better serve either the C&TS or D&S as back up power or in the case of the D&S, power for trains like the Cascade service this summer.