It stands on its own merits as a complete entity - an enlightening personal view of two of the leading watches of today.
I would nevertheless like to add my own humble perspective.
The thing which sets watches such as these apart from any time-only piece from a "brand" is their direct connection with the personality of their creators, and in this respect, the two watches are equals.
The Simplicity is, and will undoubtedly remain, Dufour's "signature" piece - his previous small output of pocket watches, minute repeaters and dual-escapement watches, while known and loved by aficionados, has been largely subsumed by the fame of this legendary series. The story behind the Simplicity reveals something of the essence of the man - a groundbreaking effort of quiet dedication to "pure" horology, eschewing the flashiness and elitism of grand complications in favour of moderation and accessibility.
It is the piece which has made PD famous to the broader world of watch-lovers and has propelled his reputation into the stratosphere. At the same time, it has proven to be something of a millstone around his neck, as (due to various well-documented personnel problems) he has been forced to denote nearly a decade of his life to the completion of this series, to the exclusion of other projects which I am sure he is dying to undertake.
KV, on the other hand, has been able to avoid this problem by restricting the output of the Observatoire to a much smaller total number. This has allowed him to continue small-scale production of complicated watches such as chronographs and minute repeaters, as well as performing a small amount of outsource work for third parties (as revealed by Gary G in his recent post). KV has also been more successful in building a team of reliable employees, allowing him a higher (although still very small - about 40 pieces per annum) production capability. He nevertheless manages to imbue each piece with a feeling that it was created by him alone.
Both men are true artists. Wearing a Simplicity or an Observatoire is like owning an oil painting by a Renaissance Master - these watches transcend mere "artisanship" to reach the status of true "art". And yet, the men behind these watches are both down-to-earth, highly approachable individuals with a conspicuous absence of airs and graces. They are both gregarious to a fault - any potential client, indeed, any watch lover, is welcome to meet them in person and get to know them as a friend, which makes the ownership experience much more personal. This, despite the fact that both men are presently overwhelmed with orders and could so easily affect an air of distant arrogance.
Essentially, this boils down to humility. Dufour is older, he has been famous for longer, and certainly he is entitled to the air of understated confidence which one detects when talking to him. But there is no brash swagger. Voutilainen is truly one of the most humble and self-effacing people I have ever had the pleasure to meet. Both men are immensely charming, and happy to talk openly and at length about their work, their lives, and their views on horology and the watch industry. This is in stark contrast to the guardedness and (frankly speaking) the evasiveness of so many in the industry when engaged at a personal level.
The Observatoire will always be famed as the piece which "broke" Kari's name and allowed him the freedom to explore his true potential. The Simplicity, on the other hand, has extra significance as a watch which (to some extent at least) has restricted Dufour's ability to create the more astonishing artworks of which we all know he is fully capable. In that sense, I would say that ownership (should I write "guardianship" instead?) of a Simplicity implies the sharing of a greater sacrifice on the part of its creator - which I would regard as a huge privilege for the lucky recipient of such a piece.
Cheers
Tony P