A lot of assumptions on this.And just to be clear: I'm not - in this thread, at least - suggesting that the Salvation Army doesn't do good work or that they shouldn't be supported.
We're off on this tangent because I pointed out to @KenS that - according to the article he linked to - "charitable" donations by Evangelicals predominantly go to churches, not to legitimate charities.
@KenS 's response of - effectively - "yeah, but the Salvation Army is a church and they do lots of charitable work" is a bit of a red herring: we can see from the financial statements he linked to that the US branches of the Salvation Army only took in $3.3 million in revenue for the most recent year where information is available. Whatever groups American Christians and churches are donating money to, it's predominantly not the Salvation Army.
... but on top of that, donations to the Salvation Army end up funding much more non-charitable stuff than would be the case if that money was donated to the average secular charity.
Regardless... Christians still give more. For all I know atheists give to charities that don't do much. (should I make that assumption too?)