To make your case, you’ll need to think carefully about your audience’s motivation.
Are there things about handwritten letters that Grandpa Dan might actually like? Are there things about e-mail that Grandma Rose might like? Does it really need to be an either/or? Can you argue for e-mail messages and handwritten letters?
What does the act of writing a letter say about the person who writes it? In what ways is a handwritten letter more personal than newer media? Does anything make handwriting less personal?
Are there aspects of e-mail that might be personal? Does e-mail have to simply be generic text on a screen? How can you personalize e-mail?