The choice depends, first of all, on the availability of a good SMTP server. If your ISP SMTP server is able to handle many messages in fast sequence, or even sending more messages simultaneously, using an SMTP server is faster and more reliable, since your SMTP server is “closer” to your computer than the single recipient’s e-mail server.
However, some providers, to prevent their users from using the server for spamming purposes, impose restrictions on the number of messages that can be sent within a specific time span and the number of messages that can be sent simultaneously (for instance, an SMTP server could accept no more than a message every 5 seconds and not allow sending more than 2 messages at a time). All messages that exceed these restrictions will be rejected with an error. This does not mean that the server or the software aren’t working properly, but rather, that it will be necessary to set in a different way the sending timing option in the Sending Options.