Which is better when using "tell a friend" - free online services or your own script? Does it even matter?
Your own script. Many of the "free" services harvest emails. I have an untested my me one for free here. Follow up in this thread whether you like it or not.
From my personal experience, I recommend you to have your Own Script. Free services irritates you in many ways.
I would suggest you to install Tell a friend script at socialtwist.com, which allows visitors to share the content without leaving the current page. This is a free service but good one. It worked well for me. Try this out.