Depends on your exchange server, if using office365 there is a limit on the number of outgoing emails that can be sent per minute with SMTP (30/min) with a direct account that goes to a reasonable use policy.
Linky to supporting stuff below:
I prefer to use EWS if possible so there are the testing features as well.
I'd agree with Alasdair, depends but I would say that it's generally easier to use EWS, especially if your in the Cloud.
I like EWS for incoming emails as it processes embedded images better and keeps them 'inline' in the Incident description field (assuming it's been set to a HTML field).