That's kind of complicated and not really a tax question.
From the non-profit's point of view, there might be certain risks to using volunteers rather than paid employees. It depends on the nature of the work, the risks, and so on. For example, if they need office help at specific times, they might rather have a paid employee than a volunteer who is under no legal obligation to show up. Or there might be insurance concerns. On the other hand, there might be benefits to using volunteers. It will be cheaper for the organization to get a $5000 donation from you than you pay you $10,000 and have you donate $15,000, because the organization has to pay employment taxes on your wages even if you return the net amount to them.
For your income tax purposes, being a paid employee might have certain benefits, like entitling you to family leave under state law, or unemployment compensation and worker's comp. Being a paid employee also gets you credits in the social security system which might be a long-term benefit for retirement or disability. But of course, you pay those taxes.
As a donor, you can't deduct a donation from which you personally benefit. Whether you decide to volunteer and donate less, or be an employee and donate back your salary, it is important that the decisions about how to spend those resources are made by other people -- the manager, board of directors, or whatever. If you were going to be the manager, and you would be in a position to decide how to spend your own donations, that puts you and the charity at increased risk, especially if you spend the money in a way that directly or indirectly benefits you (such as buying supplies from a family owned business).
If you don't donate enough overall to itemize your deductions, there is very little difference between the two arrangements financially, other than the social security and medicare tax.
