Cache Order and Priority
It doesn't really matter where you have the tags. Yes, you do save the overhead of the include by wrapping the tag, but the impact is minimal (not really measurable) and once compiled to PHP it's cached anyways.

I usually have the cache tags within my partials. It's more flexible, you can cache blocks of code separately and leave out certain parts. I also often use the globally flag and include a partial into multiple templates. Defining the cache tag within the partial makes sure I always use the right cache ID.