Agreed with what's said above... likely you, or I, or 99% of the diving population wouldn't notice the difference if their 1st stage, when tuned properly and used in recreational depths.
The MK25 is liked by tech divers due to the fact it has a 5th port, coming off the bottom, this allows some different hose routing options that tech dives would take advantage of. Both regs would be fine for the diving you do... CA is not cold enough to cause issues.
As far as an octo goes, I use a R195. It's fine for me... save the money over the cost of a S600 and go out to a nice dinner, or better yet, go diving!
At any rate is sounds like you're learning a lot and enjoying the process!
Really enjoying this process of searching and learning more about diving equipments!
Maybe not for MK17, but one of the posts I've been reading mentioned that Scubapro recommended to not use MK25 with R reg/octos. There was no explanation on it.. wondering why.
I'm leaning towards MK17+A700. Diving in Iceland is one of my bucket list so