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:
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi3.photobucket.com%2Falbums%2Fy80%2Fmpaschetto%2FSoul%2520Sister%2520Preamp%2FPC030001-1.jpg&hash=d901c7ac94df7f2491db226b80fdeda5bb047715)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi3.photobucket.com%2Falbums%2Fy80%2Fmpaschetto%2FSoul%2520Sister%2520Preamp%2FDSC04058-1.jpg&hash=1950919b39d543add80dfc01499c375e47c5316b)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi3.photobucket.com%2Falbums%2Fy80%2Fmpaschetto%2FSoul%2520Sister%2520Preamp%2FDSC04059-1.jpg&hash=0b42cb5edff8906894fed12385c6dea90e9be8a6)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi3.photobucket.com%2Falbums%2Fy80%2Fmpaschetto%2FSoul%2520Sister%2520Preamp%2FDSC04057-1.jpg&hash=1075d54b59b809db5cadc90a5391a8481a497f64)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi3.photobucket.com%2Falbums%2Fy80%2Fmpaschetto%2FSoul%2520Sister%2520Preamp%2FDSC04055-1.jpg&hash=74d61b42fb9785e087a8f1bc2f3fe3f16a25eefb)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi3.photobucket.com%2Falbums%2Fy80%2Fmpaschetto%2FSoul%2520Sister%2520Preamp%2FPA080005.jpg&hash=bda17d62c4c28ee5111e9f0f721daa4718f5b766)
Mike