Since your upgrading from the 2 to 3, do you think you'll go for the 4 next year? If so, that makes the space black expenditure to be very expensive.
Here's my $.02. I own the S0 sports watch, space gray and love the color. I the sport and stainless steel watch use the same exact internals, so you're paying a premium for the exterior. The LTE version of the spots watch uses the same ceramic rear as the stainless steel, so that only leaves the front and sapphire crystal. My glass watch face is scratch free, not sure how your's is held up. My watch body with the space gray is defect free as well.
All in all, if you buy the space black, how different looking will it be from the sport's space gray? I just don't think its worth the money, but that's just me.