How can I swap the contents of my two MacBooks? I have two MacBooks (an old one mid 2009 with a 150 GB drive (128 GB used) and a newer from 2011 with a 300 GB drive (30 GB used)) and a 1 TB Hard Disk. 

What is the easiest way to move the content of the 2009 MacBook to the 2011 model and the content of the 2011 model to the 2009 model (so that basically the content of the the two MacBooks is swapped afterwards)?
You could use Migration Assistant to move from the old to the newer, although OS X seems to have its own ideas about which OS are acceptable. Time Machine is also a good way to transfer files to a new installation. If these work this is probably the easiest.

The ultimate tool is to use the command line tool rsync which can transfer files and also (optionally) delete unmatched files. I use this to copy my user files to an external HD, for transfer to other Macs. I use a simpler rsync command to restore selected data to other Macs - e.g. I can selectively copy email data between a portable and other Mac.

NOTE rsync does not need an external disk, but I prefer the security this offers.

The following script is used to copy my user data to a mounted disk.

#!/bin/bash
# 2015-03-31
BACKUP_MOUNTED=$(mount | awk '/\/Volumes\/volname / {print $3}')
if [ $BACKUP_MOUNTED ]; then
    echo $BACKUP_MOUNTED
    echo "Commencing Backup"
    rsync -a --delete-excluded --exclude=Library/Caches ~ /Volumes/volname
    echo "Finished Backup"
else
    echo "Backup drive not available"
fi