The decision somewhat depends on the style of league you are planning on. I run a keeper league at fanball.com and we like it a lot. It's not free, but it's only $60 for the whole league with quite a bit of customization (very important for us). This year seems to be a year where many sites are going back to free, but I'm not sure what type of customzation is offered at that price. One nice thing is that many sites, including fanball.com, will allow you to setup and run the league for the first few weeks to see what you like. If you get it down to two options but can't decide, try running two sites in parallel for a bit to test them out.