Pros and cons of composing emails in clay vs. smart lead with custom field imports

Anybody know if it's better to compose your emails in clay, ready for smart lead, or compose the emails in smart lead based on custom fields being imported from clay? I assume one of the cons is that once the email copy is exported from clay to smart lead you can't change it, and may have to reupload/update leads if you want to make changes, but I'm sure there are cons to the limitations of what you can and can't do with if statements in smartlead vs inside clay (I'm already coming up against some roadblocks)