Running Docker images as Jenkins slave
In general, you can go with the assumptions that running something inside docker does not add a significant amount of resource consumption, neither CPU nor RAM, compared to starting the same process directly.

Also, it is usually good to optimize only if necessary. 

So pick the Jenkins configuration that seems cleanest/nicest/most elegant to you. If you run into trouble, then look for a solution. As you are running everything on a single physical machine, thinking too much about performance is a moot point anyways, it's not like the solution will scale in any direction, anyways. But getting to know Jenkins, and finding good configuration solutions which you may be able to apply in a "real" environment later is certainly possible in this way.