They have a good relationship with developers, which effectively means that they will have an army of people working on developing gadgets for open social.
Facebook is big, but the web is much BIGGER, and the possibilities are much bigger.
Everyone is getting hung up on getting user activity into Facebooks walled garden, and the fact that with friend connect user activity can (if the user wants it go not only in one direction but in many!

I have added Google friend connect to two blogs and my main project site; once there are some other “worthy” (I don’t mean big I just mean sites that interest me) sites with friend connect I can easily join and show my activity across all “my” (be they sites I operate, or sites I have a connection with) sites.
That is (will be) distribution, distribution, distribution!

The short sighted will choose Facebook connect. I think Facebook will re-join the bigger party in the summer of next year.

Probably the best way for you to decide which is best for is to check out some of the implementations out there. You’ll see that you can have a deeper level of integration with Facebook Connect (example: CitySearch). And you can see that it’s really easy to add signup/signin and commenting with Google Friend Connect. I suppose you could even do both. Which path is best for you depends on your site and your vision/strategy for going social. Expect lots of rapid evolution in this space, so if neither of these meets all your needs out of the box, don’t fret. Oh, and both teams are keen on feedback, so feel free to experiment and let them know what you’d want in future releases.