Hello @Tristando,
Great question!
Either method of using a series of campaigns or using flows to get your message across would work; however I would lean more towards the campaign method versus the flow method. I would propose using a series of campaigns to send your emails as opposed to compiling all the contacts you want to target into a single List to trigger a flow off for several main reasons.
By using a series of campaigns to send your message you are ensuring your target audience is who you want to target. Through using a flow, if contacts were added to the list or segment at any point they would trigger the flow. Contacts who joined the flow later can lead to contacts only receive one or two of the emails within the flow as opposed to the entire flow experience. In addition to this, if you accidentally forget to turn off the flow, the flow can potentially be sending an outdated message to your contacts.
Building off of the first point, through the use of campaigns to relay your message, you can better schedule your messages to go out at your desired date and time as opposed to flows where you do not have this flexibility and control. Comparing campaigns and flows, tracking and relaying a subsequent message to contacts who have not purchased is very similar between the two. You can do this by simply creating a segment with the definition “What someone has done, received email at least once where campaign equals X (Message 1) AND What someone has done, Placed Order at least once in the last X time frame (since the first email was sent)” or the definition of “What someone has done, received email at least once where campaign equals X (Message 1) AND What someone has done, Placed Order at least once where attributed campaign equals X (Message 1)”.
As highlighted, either method would work though and simply depends on your own personal preference and control you want over certain aspects. Some Help Center articles that may further help you choose and option are:
I hope this helps!
David