I voted green aqua terra. It's the more durable of the 2, and I believe that's a winning characteristic to have in a wedding present which signifies a timeless bond between 2 lovers.
A few things to keep in mind:
(1) Over the last 10 years, Omega's bracelets have gotten really heavy. Some like that, while me personally, I'm not a fan and prefer the lighter bracelets that Omega used to produce in the late 90's and early 2000's. If you end up getting the Aqua Terra and your fiance feels like it's cumbersome to wear due to its weight, that model really shines in how versatile it is with different watch straps.
You can pair with brown leather for the countryside, pair with a burgundy cordovan if he likes to have a splash of color, or a navy calfskin to marry together the colors of the ocean (aqua) and with the earth green dial (terra). Omega now also makes exceptional rubber straps too, if he prefers a sportier look. It'll never get old / boring, with all of the strap combinations.
(2) Omega's faux-aged lume, as found on the FOIS dial's hour markers, can feel gimmicky, after a period of ownership. When I had that artifically aged lume in a Seamaster 300 anniversary edition, the hour markers looked pumpkin orange in most natural lighting conditions.... I was not a fan. Going back to the theme of enduring, everlasting love in a wedding present... wouldn't you want the gift to have ageless white lume, rather than start out already "old and aged".... especially when it's faked aged.
(3) Service cost and chronograph reliability. There's a slightly higher service cost on chronograph movements, but when you ammortize that over 8-10 year service intervals, I'm sure it won't be a major factor. However, the day to day reliability of a chronograph movement - particular Omega's 3861 in this latest generation FOIS - won't be as bulletproof as the simpler time + date movement in the Aqua Terra. I briefly had a 3861 Speedmaster, and that would sporadically just stop when using the chronograph... others have reported that on this forum a well. In comparison, the Omega's I've had with its 8900 movement were exceptionally accurate and flawless in their operation.
(4) The Aqua Terra's 8900 movement has a hour-jump feature, which I found extremely convenient when traveling for work and traversing timezones. As a pilot, your fiance could find that useful as well!
