To keep things simple: due to some turnover, I have inherited some Marketo responsibilities. I am most often sending email blasts, however... some of these require A/B subject line testing. Any clarification would be greatly helpful.
Email programs (the "mailbox" icon) allow for A/B testing and allow for you to add a local asset, such as an email list. The Program allows you to view the Dashboard, which includes a filter for an A/B test view, if you ran one. You can also change your view to the Control Panel, which seemingly houses the same settings one would create within a Smart Campaign. The Control Panel's "Audience" box has a 'view results' option (second screenshot below) ... which leads me to my next point.
Emails via Smart Campaigns (the "lightbulb" icon) have (1.) data readily available under the "Email" tab (i.e., Sent, Delivered, % delivered, % opened, % clicked, etc. etc.). and (2.) Compared to Email Programs, Smart Campaigns also have the same "data dump" metrics under a different "Results" tab. (See first image below)
We use default programs for email sends (vs. separate email programs) for about 90% of time. Simply because of the flexibility that you have (multiple wave sends - which field marketers love); easier reporting; no confusing naming conventions (of the email itself) that exist when using A/B testing in email program; etc.). The only time we will use an Email Program is when we want to do automated A/B testing (and we have a large enough audience) or for some of our internal campaigns that we know will only be sent once on a regular cadence). Ideally, we would love for default programs to support automated A/B testing of batch emails (vs. Champion/Challenger). Also, Email Programs aren't supported in Engagement Programs. We use child programs in all of our EPs.
Rather than repeat what has already been shared on the community, have a look at these existing posts: