Definitely the 3861! I upgraded a couple weekends ago, and haven't looked back.
Major Pros for me:
1) Comfortable caseback, if you're going with Hesalite. The hippocampus is now recessed into the caseback, and no longer protrudes higher than the flat plain of the caseback surface. The sharp Seahorse on my 1861 was so uncomfortable and left deep red marks on my wrist even after just an hour of wear... that I had to swap it out for a Thomas Priek display back for the watch to even be tolerable for wear.
2) Accuracy. My 3861 has gained a total of 24 seconds over 15 days. +1.7 seconds per day! In comparison, 2 different 1861-powered I had (last generation Pro, FOIS) each gained
10+ seconds every day. I personally found it annoying have to rewind the minute hand back every 4-5 days on my 1861 Speedmasters, just for them to display the correct minute
3) Hacking seconds
4) Crown is easier to wind <-- this quality of improvement adds up, given you'll be winding it everyday.
5) Bracelet comfort.
6) Smaller box. I never cared for the loupe, paper weight, etc... and the NATO + astronaut straps never left their wrappers in my 4 years of ownership. Plus the 3861 Speedy comes with a built-in travel box.
7) Stepped dial. Looks very sharp in direct sunlight!
8) The white markers in between seconds match the beat rate of the movement.
Things I miss about the 1861 Speedy Pro:
1) Dial text pyramid. Together with a smaller & lower teardrop on the chrono hand, the symmetry and linear slope of 1861 dial's text is very beautiful to behold.
2) Clasp. I find the new generation's clasp much too fussy, with the sculpted logo and ribbing. Plus the 3861 bracelet's final link looks terrible if you're not using the interior micro adjustment slot in the new clasp.
3) Beefier chrono pushers. Haha, purely a personal aesthetic preference.
4) Price
Have you seen the Hesalite in person? The warm distortion of the plastic crystal is what
makes the moonwatch, in my eyes. There are many collectors who are passionate Hesalite versus sapphire. For some, it's important that Hesalite is the very technology used in the original moonwatch. For me, I like how it has a connection with other vintage watches (like the fan-favorite pie-pan Connie) before sapphire became the norm on luxury watches. What I enjoy most about the hesalite, is all about how cool the distortion effect looks!
Not a popular opinion on this forum, but I've never thought Omega's movements are all that pretty. I'd gladly replace the the sapphire display backs on my Planet Ocean and Globemaster for a plain solid caseback to reduce the watch weight & case height. I can gawk at a 321 all day on videos, but personally find the 1861 and 3861 a little boring. On a manual-wind Breguet, Vacheron Constantin, or Lange? Sign me up for a display caseback. But on bread & butter workhorse movements used across the model ranges in brands like Omega, Rolex, Breitling, IWC, my personal preference is to get a plain solid caseback. Will also add that it's only a matter of time before after-market display casebacks are available for the 3861 Hesalite Speedmasters.
Have fun picking out your Speedmaster at the AD! Also recommend getting it on bracelet, as it's much cheaper to get strap + buckles a la carte than it is to order a bracelet later on. The new leather strap on the 3861 Sapphire Sandwich is very boring compared to the alligator strap on the previous-gen 1861 Sapphire. Plus, you'll find many threads on here exhibiting really nice OEM straps, and you can build a solid collection of very high quality after-market straps for the price of a single Omega-branded one.
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