SMTP vs Exchange Web Services

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:

https://support.office.com/en-us/article/How-to-set-up-a-multifunction-device-or-application-to-send-email-using-Office-365-69f58e99-c550-4274-ad18-c805d654b4c4

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).