The difference.

Every "traditional catholic" is aware of at least 2 "armies" fighting for the Tradition of the Church. First of them is FSSPX, the latter - all convents, priesthoods, institutes etc. gathered under the banner of Ecclesia Dei. Despite one goal, restoration of the Catholicism, there are sometimes clashes between them. One, long lasting, is about being faithful to the Supreme Pontiff. Priests and well-wishers of IBP or FSSP could say, that FSSPX is movement, which slowly becomes a sect, influenced by "ghetto mentality". Why? Because they still vehemently oppose to any form of agreement with the Holy See, which involves consent on any doctrinal issues presented  in the spirit of the Vatican II. They love Catholicism. But FSSPs and IBPs etc. also love it and want to preserve it. They are allowed to celebrate Masses and other Sacraments according to the usus antiquior being in the same time reconciled with Rome. Outstanding, is it not? So why FSSPX opts for fight? Because they are not interested only in the Mass itself. Of course this is the core, but what about other "layers"? Is it possible to glorify ecumenism presented in Ut unum sint, simultaneously presenting the idea of Ecclesia militans or dogma Extra Ecclesiam nulla salus? No. This is the crucial factor in understanding the Brotherhood's resistance. It fights to restore the whole Catholicism: Liturgy, theology, education system, political and economical concepts. Is that equal with the statement, that Ecclesia Dei institutes are worse/naive/pointless? No. They have their task to complete for the Holy Church. They popularize Tradition, maybe more efficiently than the Brotherhood does, because their past is "less turbulent" and they are allowed to act in ordinary parishes, but on the other hand they are completely subjected to the local hierarch, who may block the actions of such ministry (what naturally happens e.g. in South America). What is my position? I am a "spiritual member" (because I am not a priest) of the Brotherhood of Saint Pius X, but I do not despise other communities longing for truly Civitas Dei in the world. As the Holy Gospel says  "Qui enim non est adversum nos, pro nobis est" (Vulgata Nova; Mk 9,40), it means "Who is not against us, is with us" I strongly believe the more are we united, the more Tradition will flourish. Eventually the struggle is not for names, rites or concepts. It is for the survival of the Catholic Faith, the Faith given to us by God himself, which must remain pure. And it is our duty now. As a conclusion and confirmation of my words I would like to quote Servant of God archbishop Fulton Sheen (1895-1979), who once said: "Whom we may rely on in the fight to save our Church? Not on our bishops, our priests or members of religious orders. It's your job, faithful layman ! God gave you reason, eyes and ears to save the Church. Your mission is to keep priests behaving like priests, bishops like bishops and friars like friars".