I was briefly considering a set to run as winter wheels, but in the end I decided to stick with my black Neptune's in the same size and offset.
Weight is similar to the standard 18 inch alloys AFAIK as they’re cast. Don't think size-wise they'll look too bad if your existing wheels are 18 inch, but they're obviously not going to fill the arches as well as the 19 inch version.
Colour-wise i'd go for black, as the silver isn't the same shade as the 19 version. I think the 18 is a brighter shade of silver than the 19 inch version.