Upgrading from 6 to 7 (which you will have to do when they end support for 6) is painful. I had a bunch of errors using stable releases of 7 upgrading from 6. I would HIGHLY recommend using 7 right out of the box. Some modules don't work, but there are plenty more that do. Meta tags is one of the modules holding up users in 6, and it's really not helpful in increasing your SEO anyway, so make sure you MUST have a module before choosing to stay in 6. The interface of 7 is better and the update system is a vast improvement over 6. Theming and module development is mostly the same, only a few variables and functions changed. In my mind, 7 is the clear winner. It's like how WinXP uses less resources but Win7 improved in so many areas, they're not even comparable.
