The S-1000 offers the all aluminum stand, Euro bracing and a slightly larger sump and display volume. Is that worth the ~$1800 in price? Maybe.The updates with V3 and the 900 definitely made me feel better.
I am curious though, if size wasn't an issue for your basement, Why would you go with the S-1000 at $6600 with the current $1800 in savings with 900 being at $4850? From a structural standpoint I'm not really worried either way. The Euro bracing/trim is nice as it will keep some critters and fish in the tank.
The volumes are much more similar than pricing. So the real question, is the extra cash for all aluminum and euro-bracing worth it due to the structural security even after the updates for you? As someone with the V3 I feel like you're my best bet here lol. There so much recent Red Sea hate going on right now I can't get a solid opinion.
Me, I would get it just for the aluminum stand alone - but that's just me. The Euro bracing/trim also helps keeps various critters and fish in the tank.
I haven't tried the Neptune SKY but I've heard good things about it.