sam1234
358
Another version to consider, if rarity is a factor,
Would be to locate a 3940 with the dore dial. I believe those to be quite rare as well, though certainly not so limited as Chimaera's example above.
I'd agree that the first series of the 3940 does seem to be the collector's choice, but I much prefer the second series with separation of the years. It makes the dial easier to read, though some might say less balanced. The third series had further changes, particularly with the font, which I generally find less appealing.
The 5140 will almost always be more expensive, metal for metal, as it is the newer and slightly larger version (the 3940 was discontinued in 2007 iirc). The retail price was higher for the 5140 then the 3940 throughout its run, and likely the second hand prices at least somewhat account for this as well.
Both are great watches, use the same reliable movement, and are essentially identical but for the aesthetics and 1mm size. Rarely do we hear of mechanical problems with either of these watches, perhaps a function of better previous QC, better design, or simply fewer folks with access to the internet then there are today looking to vent their grievances.