A family of Microsoft relational database management systems designed for ease of use.
DoCmd.SendObject is "native" Access code. You can attach an Access object (table, query, report), specify the recipients, subject and message text, and whether the message will be displayed or sent immediately. You can't specify formatted text, and you can't attach other files. It will send through the user's default e-mail client.
Automating Outlook through CreateObject("Outlook.Application") or GetObject(, "Outlook.Application") starts or activates Outlook. It offers more options, such as specifying formatted message text, and attaching arbitrary files. You can specify whether the message will be displayed or sent immediately: the MailItem object has .Display and .Send methods.
If you don't need the extra options that automating Outlook offers, I'd use DoCmd.SendObject - it's more general and less likely to cause problems because you don't start another application.