Sounds like there's 3 important parts of your equation: (a) how the watch makes you feel, (b) magnetism, and (c) which is a better buy at those prices.
Having owned a FOIS and a standard 1861 Speedy Pro, my vote is for the case shape of the standard Pro. It's a personal preference, but I've always liked Omega's lyre lugs. Between both a FOIS and a 60th anniversary Seamaster 300, I found straight lugs boring. The unprotected crown on the FOIS is so, so, sooo nice to wind though!
For magnetism: my FOIS never got magnetized, but I've had other watches with more delicate watch movements get magnetized frequently. A few different OB's have told me 1861 Speedmasters are the most common watches they see come in for de-magnetization, but wouldn't worry about it in any modern Speedmaster though. Even if it does get magnetized, it's a quick trip to an AD or boutique and they'll de-mag it in a minute or 2.
Accuracy-wise, both of my 1861's run about 10+ seconds every single day. This is my primary reason for wanting to upgrade to a 3861. For a luxury watch in these price ranges, I don't want to put up with the user experience of having to set my minute hand back 1 minute every few days. People will say "get it regulated".... but I'm of the school that prefers a product to work optimally out-of-the-box, than have to take something in for tuning as soon as I buy it. If I'm an equipment/procurement manager at NASA back in the 60's, I'd order the watch rated to keep chronometer specs out of the box, than a watch I'd have to test on every delivery, and then go through the chore of pulling watches out of each batch to a local watchmaker to regulate just so it doesn't gain a minute every few days.
Price-wise, I think that's a terrible price proposition for a pre-owned FOIS. These were going for 3,200 USD (2,300 GPB), brand new, out of the box when I got mine ~3 years ago. I'm sure you can find some forum members who are happily willing to part with theirs for less than the price you're being quoted.
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