As one considering a SMP 300 Master, I have followed this thread with considerable interest and wish to jump in with some comments.
First let me say that the Submariner is not now, nor has it ever been, on my radar. That is not to say that I am not a Rolex person. I have owned serveral including a 16600 Sea-Dweller which IMO is only Rolex diver to own. I have also owned a GMTIIc, a GMT Master, a Datejust, and a President. Currently in my collection is a polar 42mm EXPII which is about as good as it gets.
However over the years, I have probably owned over a hundred Omega watches from vintage to modern so I feel somewhat qualified to address some of the issues raised.
As to the qualities of SS, from experience I can say for certain that the 316 is more scratch resistant than the 904. I had a Rolex and an Omega on the winder next to each other. Unfortunately they were too close and actually rubbed against each other for several days. The case of the Rolex was noticeably worn from the friction but the Omega case was unblemished. But as a general rule, the quality of the SS is a non-issue, at least for me.
Movement-wise, my Rolex and Omega watches are all good time keepers. Rolex fans no longer have the argument regarding in-house versus "out-house" which has been irrelevant since Omega has owned ETA and Lemania for years. Rolex has gradually improved their movements over the years and I have no complaints. But Omega's new movements make (at least theoretically) a giant leap in watchmaking. Early co-axial movements were based upon standard ETA movements and had their issues. Fortunately for me, none of the seven cal. 2500 co-axial watches I owned ever failed and all were excellent timekeepers. I've owned three 8500 movements, two AT's and a PO, and found them to feature rock solid accuracy and I particularly liked the hour-advance and 60hr reserve. Omega touts the extended service interval of the co-axial but I have never owned one long enough to comment. I will say the co-axial movements will be more expensive to repair and service because of the watchmaker's cost of the special equipment and extra training required. Many watchmakers are unwilling to spend the extra time and money so your local watchmaker may leave you "out in the cold."
A major factor in my watch buying is comfort - wear ability on my wrist. I have a 7 1/2 inch round wrist and thick watches tend sit precariously and slip from side to side. This was not a problem with my cal. 2500 POs but the 8500 was like a giant wart on my wrist and no where near as stable.
Most comments by those owning both the 300 and 2500 POs find the thickness of the two watches to be comparable. The comments about the extended lug length of the 300 does cause me some concern as my round wrist is not that wide. A trip to the Boutique may be in order to resolve that issue.
Now to address design and styling. Like with my "First Omega in Space," Omega wanted to update the Speedy Pro with current advances (rhodium plated movement/hands and sapphire crystal) while playing homage to the vintage look of the Speedmaster Schirra worn on this Mercury flight. Admittedly, I had issues with the FOIS design, particularly the sapphire crystal, but I eventually replaced my vintage 105.012 "Neil watch" with the FOIS. I'm an old guy and appreciate old (and old-looking) things and I guess that is why I am considering the 300. I think it will pair nicely with the FOIS. Yes, the bezel is narrow (but state of the art) and the dial is not modernized with applied markers but the intent was to capture the look of the original as Omega did with the FOIS. PCL's are not my cup of tea but my watchmaker can remedy that.
For me, as it seems for others, the bottom line is how it wears. Stay tuned.