I do not know if it helps with anything, but this is what I wrote in another discussion about the differences between the FSSP and ICRSS (I don’t know if the info on the ICRSS will help or not… but here it is):
"The FSSP are closer to diocesan priests… they are in many different dioceses, but the bishops in those dioceses don’t have much authority over them… they are a Society of Apostolic Life of Pontifical Right, so they only have to answer to the Pope pretty much, but they do have a Superior General and whatnot. They do not take religious vows as they are not a religious order. They do however have a group of Discalced Carmelite nuns somewhat attached to them… an FSSP priest is their chaplain in PA. They do not seem to own very many of their own parishes, they are usually welcomed by a bishop to take over a parish that would be dedicated to the FSSP, but the parishes they receive are not usually the most ideal, they have to work with what they get… I guess the better way to put it is to say that they receive the poorer parish buildings and whatnot, but they do a great job with connecting with the people, teaching classes on the Latin language, teaching apologetic, etc. They were founded when some SSPX priests left the SSPX because the SSPX bishop ordained some priests to the episcopate without papal approval, thus excommunicating the society… these SSPX priests ran to the Holy See for aid… Pope John Paul II saw that they were sincere in wanting to avoid excommunication and so not to be in disobedience of the Holy Father, so the Holy Father helped get the FSSP going. The FSSP has two seminaries, one in Wigratzbad, Germany (for German, French, and Italian speakers), and one in Denton, Nebraska, USA (for the English speakers). Their charism is titled “traditional”
fssp.org/index.htm and
fssp.com/
The Institute of Christ the King Sovereign Priest seems a bit closer to a Canons Regular or a religious order (I hear that there was talk of them becoming a religious order, but I have no confirmation on this), but they are still similar to diocesan priests. They are in fewer dioceses, but they are a bit newer, but the bishops in those dioceses don’t have much authority over them… they are a Society of Apostolic Life of Pontifical Right, so they only have to answer to the Pope pretty much, but they do have a Prior General and whatnot. They do not take religious vows as they are not a religious order. However, they do have an order of sisters: “Adorers of the Royal Heart of Jesus”. They are welcomed into various diocese, yet fewer dioceses in around the world… and their parishes are a bit nicer. They were founded in Africa and brought over to Italy because many people had a traditional charism. They do have parishes in the U.S., but their only seminary is in Gricigliano, Italy where you may have to speak French? However, they do have an oratory in America (St. Francis de Sales Oratory) in St. Louis, Missouri with a pre-seminary program. They also seem to have a sort of “habit” (usually blue or black)… similar to the blue habit of their sisters. They also seem to use more lace.
institute-christ-king.org/ and
institute-christ-king.org/stlouis/
I hope that helps.
Pax Domini tecum."
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http://forums.catholic-questions.org/showthread.php?t=394769
I might also add that they have different patrons… the FSSP def. has St. Peter and probably unofficially–Pope Pius X and such, while the ICRSS has St. Benedict, St. Thomas Aquinas, and St. Francis de Sales.
It’s my mother tongue.