Hi Mike,
I've built all of the preamps you mentioned, as well as one you didn't....an Extended Foreplay 3.
To answer your first question, I'd rank the preamps in the following order:
1. Extended Foreplay 3
2. Soul Sister
3. Foreplay 3
4. Foreplay 2 with C4S
Please bear in mind that my list is subjective, and you may well prefer something different than me.
To answer your second question; yes, the Soul Sister is certainly a worthwhile project!
It is easy to build, sounds great, and the B7 transformer output can be configured for balanced output
if you'd like. I tried it both ways, and the preamp was dead quiet in each configuration.
Here are a few photos:
(http://i3.photobucket.com/albums/y80/mpaschetto/Soul%20Sister%20Preamp/PC030001-1.jpg)
(http://i3.photobucket.com/albums/y80/mpaschetto/Soul%20Sister%20Preamp/DSC04058-1.jpg)
(http://i3.photobucket.com/albums/y80/mpaschetto/Soul%20Sister%20Preamp/DSC04059-1.jpg)
(http://i3.photobucket.com/albums/y80/mpaschetto/Soul%20Sister%20Preamp/DSC04057-1.jpg)
(http://i3.photobucket.com/albums/y80/mpaschetto/Soul%20Sister%20Preamp/DSC04055-1.jpg)
(http://i3.photobucket.com/albums/y80/mpaschetto/Soul%20Sister%20Preamp/PA080005.jpg)
Mike