When you get first blood as a support

I think the latter is nicer.

If a support can rush sightstone AND get their second tier support item (especially coin!), they get more gold income as well as safety early on.

Early game, typically it's the adc's job to farm farm farm and not focus too much on killing. Assists still give gold, and having one less lane opponent for a few seconds can help your adc get dat gold advantage.