My understanding is that one of the key advantages that Ariane has over it's rivals is it's launch site near the equator (http://www.arianespace.com/spaceport-intro/overview.asp). Is this saving negated by other factors, when compared to SpaceX?
That's only an incremental advantage. Note that SpaceX also has an equatorial launch site but they haven't used it since the Falcon 1 because it's just not as practical as operating out of Florida/California.
Comparatively, SpaceX is operating a much simpler rocket that is manufactured much less expensively than the Ariane 5, by about a factor of 10 or so.
As far as centrifugal advantage, SpaceX gets 88% as much launching from KSC in Florida. There's also an advantage for geostationary satellites (you avoid an inclination burn) but I'm not sure how large that would be.
