There's a couple issues I want to raise, first starting with the idea that people who are religious are "charitable" than those who are not, a couple of statistics:
A 2007 study by The Barna Group found that “active-faith” individuals gave on average $1,500 in 2006, while “no-faith” individuals gave on average $200. “Active-faith” adults gave twice as much to non-church-related charities as “no-faith” individuals.
I know what you're thinking, wow, non-religious people are doing a really poor job of donating to charity even when it comes to donating to "true charities" which have nothing to do with their religious causes. It appears that way, but then look at this:
In 2005, people in the 65-74 years age bracket gave the most dollars to charitable contributions. The people in the 75 years and older age bracket gave the highest portion of income. Percent of total income given by age:
All consumers Under 25 25-34 35-44 45-54 55-64 65-74 75+
1.7% 0.7% 1.2% 1.4% 1.6% 2.1% 3.0% 3.7%
Why is this relevant?
I think this collection of statistics shows that it isn't that the religious are more likely to give more of their income. It's simply that the Elderly give the most, the Elderly tend to be predominantly religious, and so the numbers are inflated in favor of the religious. I think the only way it can be proven that the religious are more charitable is if they did an age bracket by age bracket comparison of those who are not religious and those who were to get rid of the back-loaded magnifying effect.
The other issue I wanted to discuss was this notion of having giving to religious organizations be considered "charitable contributions." If when the offering plate is passed by Sunday morning, why should you be able to write it off on your taxes if you toss some money in? You're paying for the services to support your religious organization. It makes absolutely no sense to me why you should be able to get a tax break for that.
This also ties into politics in the United States. Conservatives love to use that they give more to charity than Liberals in various arguments about the Welfare State. The statistics are, again, skewed because conservatives tend to be religious (which means their contributions to their churches count) and older (which means they're in the more 'comfortable' giving bracket).
EDIT: For the sake of a summary, let me state my argument, I think it'll make my logic a bit clearer:
The information states that if you take the average amount given to secular charities by the religious and the average amount given to secular charities by the non-religious, the amount given by the religious is more. I contend that this has to do with the fact that the religious tend to belong in greater numbers to portions of the populace that are in a position to give more money and has nothing to do with generosity.
I think if they did where the average non-religious individual in each age bracket was compared to the average religious individual, then we'd have an idea. The problem is the religious population is more densely packed in the age groups that naturally give more and the non-religious population is more densely packed in the groups that give less. That's going to mess up the results.