Personally I would go with .org just to give some sort of semi-professional feel to the domain name. That said, if you can afford both domains, I would register both of them. If you look around for GoDaddy or other registrar coupons you could probably register both domains and that way avoid some domain squatter from aquiring your domain and charging you and arm and a leg years from now.
Yes, it depends on the name and your needs... But to have an idea of their value, you can check 3character.com/priceguide.html for 3 characterrs and dnsaleprice.com for previous sales...
.org has historically been for non-profit organizations. If you are planning on selling products or services from your site, I would suggest .net
If it's for an organization or non profit than of course go with org. However if it's simply a choice of .org or .net, I'd go woth .net
.org ranks higher much easier than the rest of the domains (other than .com or country specific domains of course) in my experience -