I suppose it is more reliable for Elk to send an email about an Elk event. It obviously means that only the Elk needs to be working and not the ISY. However, ISY is much much better at doing the email thing. Wayyyyy more things you can do, plus SSL support. Or really you can just use ISY's mail server and not worry about any settings.
If you have the ISY Elk module, you can have ISY include all kinds of variables, like zone status, voltage, armed status, alarm status, and the list goes on. Emails in Elk are fixed in content and you only have 16 total contents. ISY lets you have a gazillion (actually I don't know the limit off the top of my head but it is way the heck more than most any human might actually need).
Elk requires that you use a non-ssl email server. Virtually every free email service only accepts ssl. So you are probably not getting your test messages because you tried to use an ssl only server. And my Elk is the most finicky thing ever. It crashes the entire xep unit if I put a port number or server name that it isn't compatible with and hit "test".
