This choice seems to depend on what we think is our priority.
I think settling the silver site with Washington's settler would help our early research/commerce given the nice trade off of the gold with stone city and helping to get the silver up faster. Plus the city will be getting the extra trade routes from the GLH by the time they get settled. So this would offset the earlier increase maintenance that settling them earlier would cause. Additionally a settler from gems could possibly be produced to settle the horse site reasonably soon (although this might delay a great scientist assuming this is what we want next in terms of great people). What would the settling date be for a settler produced by gems? T82?
The horses site first does help guarantee access to the horses and bronze from both AI threats (which I would have to assume is quite low) and from barbarian threats. The barbarian city threat will be reduced significantly with the arrival of our warrior. However he could be driven off by a barbarian archer. A barbarian archer could possibly be avoided with a settler if one does appear, but I can see how a single archer would block us. The maintenance for this city will be somewhat greater. Not sure how much of a factor that will be since we clearly want to settle here soon. Guaranteeing access to horses and copper might have a significant impact.
I think if we can get a settler out of gems in a reasonably time frame I would vote to settle the settler from Washington near the silver first.
I think failure in securing the strategic resources is worse than the benefit of getting the silver site. It seems to be a small gamble, but if it all works out, the silver site 1st has a bigger payoff.
We should also have a great spy walking through that area and get to the horses at T83 or better.