[Fred]

Nice and good tutorial. Some remarks on the tutorial:
You forgot (or I missed?) the step were transmission is installed, or does transmissionrpc also include the tranmission client itself? If yes, is there a way the xbian-package of transmission could be used? Since this one is patched for XBian and compiled specially (lightweight) for the RPi.

At step C, the folders /home/xbian/downloads etc are create when the download package (of on of its includes) is installed. So won't be there for default (they will be when xbian-package-transmission is installed).

At the extra: If you enable portforwarding it's recommended to change the password for transmission also. It is also not recommended to enable port forwarding for port 22, because you will get several brute force attacks per day on that port. If you want remote ssh access from everywhere best is to change the ssh port number (google for more info on this).

Flexget also offers support for newsservers, so you might want to add that to your tutorial.

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Some remarks on the claims you made in the beginning. The claims to more relate to torrents vs newsservers then FlexGet vs SickBeard/CouchPotato.
CouchPotato perfectly supports torrents, so no need for a newsserver.
SickBeard offers limited support for torrents, but there is also a fork with full support (which is also offered as an XBian package). So again, no need for a newsserver.
The parring and extracting is related to using newsservers.
The last claim about prioritising feature included in >XBian beta 1 so can be applied for any package, also for SickBeard and CouchPotato (which it also is). So these can also run simultanious with XBMC.

The way I see it:
Flexget:
pro:
Lightweight.
One solution for both tvshows and movies.

con:
More advanced to install and configure.

CouchPotato/SickBeard:
pro:
Easier to install and configure.
Full and fancy GUI.

con:
Not so lightweight.