I would say choose what your going to be exporting with; and how necessary the slow-mo is. If your going to be putting it on the web, and exporting it into 720 anyway for a smaller file size, might as well shoot in 720 and be able to shoot more on that card too. Also if the slow motion is really helping your project thats another factor. However, if the extra resolution is worth it to your project, you gotta make the decision, whats worth it to you, and will you be using the gain