The 687 doesn't sing to me. If you are looking for a Radiomir, there are far better options at that price range or cheaper. To name a few in my order of preference: 249, 190, 232 to name a few. Getting a 249 might be a bit tougher than the other two. Not to mention, 721 and 722 will be released soon and I'd choose the 721 over 687. I was offered a 685 two weeks ago and I never even asked to be on the waitlist for the watch. I can think of one other person who bought a 685 and immediately turned around and put it in the market.
If you are looking from the perspective of an investment, I would go with the new SD over the 687. A 50th anniversary SD will carry much weight in the future. Having said all that, it comes down to your preference and what you are looking for. Given that you were never looking for the 687 but is looking forward to the SD, it sounds like you are looking for confirmation from us not to get the 687. I think you already know what you want to do. Pull the trigger on the one you initially had your sights on.