Among the 2 listed above, try Comodo first. Note that these certificates help:
1. Verify your identity
2. Encrypt email.
However, since these are free and your identity is not verified before issue, they can be used only to show that the email ID you used was the one that is the originator. If you are happy with the way it works, you can then apply for the real certificate with any Certifying Authority (including Comodo).