Well my 14 days is up and after wearing both the Space Grey and the SS for a couple of days at a time each, I've finally decided to keep the SG Sport and return the SS. There's no doubt the SS is nice watch - premium in several ways - but for me I prefer the more casual look/feel of the SGS and feel more comfortable wearing the SGS, so for me the extra £180 is not worth it, but for others it absolutely would be.
The extra weight of the SS was noticeable when running/exercising, which meant it had a tendency to slip down my wrist more than the SS, the sapphire screen is not easy to read in daylight and I miss lots more taptic notifications on the SS than I do with the Sport. And despite the SS being more robust its looks so precious I was more conscious of not knocking it as I would probably cry if I did! Also, I think the current rumours of version 2 of the watch also reminded me that this is a generation one model and I'm happier spending less now and maybe upgrading next time round.
That's my two'pennth - everyone is different, the SS is definitely a beautiful device so whichever you choose, just enjoy!