The common consensus is that the 3940, especially 1st series, has it all - historical interest, "the" classic PP DNA, beautiful balance/clarity and probably the most useful complication of them all. I suspect that the apparently lower value has to do with evolving fashions, no more - the case size can seem small.
If you are looking to build a collection, and your wrist size is not too large I would say 3940 all the way.
I also like the 5140, especially with blue face. The critique of that is that the dial is not as clean, especially the date ring with two "squashed" numerals at the intersections with the upper sub-dials. This doesn't bother me overly, but it does others. I would very much suggest you see both in the metal before deciding, because the two issues (3940 case size, 5140 script) cannot be assessed from photos.
If you are interested in long-term value, neither are rare. On this metric you could seek out a rare London Edition 3940. Here is a pic of mine. One one view it suffers from both above issues as you can see the squashed date numerals but for me it works because there are other differences e.g. The finer hands, and readability and "class" are evident. I believe there are only 5 of these in existence but David Duggan in London did have one for sale a few months ago.
(Can't upload photo, will try later).