Well, I gotta go against the grain here....
I'm going to say "depends on what purpose" - I think a 3.5 in 27 would be a lot to carry. That said, a 4in 66 isn't that great either, but it is better.
In general, I would buy a 27 in 3.5 in, esp because thats a premium barrel length. If I wanted a gun for even occasional carry or better yet, primary carry, I would pick the 66.
Financially speaking, an $800 OTD 27-2 with 3.5 in barrel is probably a better deal. They are worth at least that much. A 66-1 is much less desireable.
Also, yes IMO a nickel 27-2 3.5 in would be snapped up first and they are normally more money as well.
Winchester 73, the TFL user that won the west