Thank you for your response.
Technically, it's not a problem making either of the two options and making sure Googlebot is served the correct version. What I'm curious about is the SEO effect of using the same URLs and how it will affect usability. I know I always get a bit annoyed when someone sends me a link from their mobile that sends me to the mobile-friendly version and I'm on a desktop.
Also, what about external and offline advertising where you want to link to domain.com/product. Would it be better to redirect or cloak?