I would go for .net but if your site has something to with nonprofit/organization then I think it just make sense to have .org
If I really liked a name but the .com was taken I would actually look at other tlds before going to .net or .org these days. A tld such as .io or .me would be better for my needs
.net, but I would seriously consider thinking outside the box for a .com domain name, or even look into expired domains or domains that are currently for sale for a quality .com extension. I have had sites with all 3 as an extension, and .org used to work extremely well for me about 5 years ago, but has since lacked. I now only focus on .com, mainly because of visitors who want to manually enter in your domain name. if it is a .net or .org, you better believe there will be a percentage of people who type in your domain with a .com anyways! just my $0.02