Need advice on d6-d7 Upgrade

I'm stumped on the verison of Zen to use on my upgrade path. I currently have a subtheme based on 6.x.1 on my existing site, and realize I'm going to be basically recoding a new sub theme, but which should I choose for the upgrade. I know the difference is xhtml 7.x.3 vs. html5 7.x.5, would it be wise to do a major version upgrade and an upgrade to the theme language too? What are the problems I'll face by switching to html5 now???