Don't know if there is any significant speed differences, but there is a known bug associated with SMTP emails that can cause them to be mis-identified as Spam by many spam or junk mail filters.
For More Information see: Email header malformed when using SendMail
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
Can't believe this issue has been unresolved for 2 years. That's rediculous :(