I generally think of our email systems like this: Marketo for mass promotions and driving response to an event and Splash for all comms after someone has converted (registered) for the event.
Promoting via Marketo allows for advanced segmentation and ensures communication preferences are followed (i.e. what kind of content someone wants to get, how often they receive communications from the company, etc) without having to move/manipulate data outside of its primary platform.
Once someone has converted and the emails are more transactional in nature (reminders, bring a friend, post-event, etc) it makes sense for communications to come from the place that captured that data -- Splash!
Important to note: Even when we do send emails from Marketo, we always design them in Splash (usually from a template) and export to HTML -- it's so much easier than starting from scratch or a bad Marketo template.
Also, emailing from Splash makes me despise certain things about Marketo -- no emojis in subject lines or body -- Boo! 😣