Fanpages gets more traffic cause it`s not much to think about when you come pass something you like... you just hit like, and move on. People have to think more on the choice of joining groups. Like is it the right group,who is in the group, what are they talking about in the group etc... If you use facebook ads to promote fanpages you deff will get some awesome traffic and fanpage leads .
I don't think you want to limit yourself. You can automate some of the content posting. For example, I have my blog updates posted on my fan page wall instantly. You can also use groups as an extra traffic tool. There are so many ways to use Facebook, and automate some of it that it would be a bad idea not to take advantage. I also use the "import notes" tool to get my rss feeds posted on my actual wall. Facebook is my #1 social media traffic source too.
I think it depends on your purpose. Groups tend to work better when you have a lot of similar people - like people that work in the same place. Groups are also good if you need to be private. Pages are for the public and anything YOU (the admin) post will show up on Fans/Likers newsfeed and they can share the content easier. Groups get notifications though. I like them both. And honestly - I pay more attention to the groups I am in than the Pages I Like.
Groups are often a best way to interact with people and promoting a business first hand. But fanpages offers a lot of features to gain a lot more traffic.
fan pages obviously. from your like button you can get a huge number of fans in your page. also it is ethical.
Just a note, Facebook is restructuring it's groups in November, and all members will have to re-join afterwards. If you have any large, responsive groups, I would build a similar fan page and start getting members to migrate over.