Here's some info I gleaned from a google search:
From
http://www.salt-city.org/showthread.php?t=140
Here are the advantages of the Jr over the RK2:
- Can control up to 12 devices. RK2 only controls 8.
- Seasonal lighting variation. Not present on RK2.
- Seasonal temperature variation. Not present on RK2.
- Moon Cycle simulation. Not present on RK2.
- Datalogging of pH and temperature. Not present on RK2.
- Temperature compensation of pH. Not present on RK2.
- Much more flexible configuration on the Jr.
- Jr less expensive $250 vs. $270 for RK2 equivalent configuration.
On the Jr w/ the serial port option these additional features are available (none present on the RK2):
- Computer Interface so that it may be connected to a PC.
- Digital input. (can connect to float switch, water on floor sensor, power fail sensor, etc.)
- With AquaNotes Lite (free)
- datalogs are stored in the database.
- Graph of the datalogs.
- configure controller through PC.
- With the Full version of AquaNotes
- Email alarms.
- Web access of the controller.
- Event logging.
Another link
http://reefcentral.com/forums/showthread.php?s=&threadid=825724&perpage=10&pagenumber=1