The Orthodox and the RCs agree that the True Church has four marks: it's One (which means it can be only one group although having sub-groups; also it means it is undivided), Holy, Apostolic and Catholic.
Each group will define these words in more or less different manners. Protestants, on the other hand, have a wider more blurred concept of "True Church" where acceptance of Jesus Christ through an interpretation of His life in the Bible is the great unifier.
For the RC, One means primarily being one institution, which would guarantee that it would keep one faith. When something cannot be clearly defined philosophically or theologically, the highest institution in the church, that of the Pope, through its current pontifice can, ex cathedra, by inspiration of God, point, define and proclaim which of the diverging opinions is the true one, that none is, or what is right in each and form a new more perfect one. Ex Cathedra is a generic concept that means literally "from the throne" and in practice "based on the spiritual experience of the Church in this and all times". He can't say anything preposterous, but if it can be justified, even with the concept that it had been implicitily believed from the very beginning, it will do.
For the Orthodox, One means primarily the same faith, synthesized in the Symbol of Faith and expressed in a plethora of ways, since liturgical traditions,hymns, ascetic practices etc. The Orthodox not only allows but in fact leads to the existance of multiple eclesiastical institutions, which should be attached and more or less equivalent to social divisions: the local parish, the regional diocese (until here the RC agrees with us) the inter-regional or national archdiocese or metropolia, among which the most proeminent ecclesiastically and politically can become autocephalous (self-governing) and, if *very* proeminent and usually with a couple of centuries being Orthodox, receive the title of Patriarchate. For the RC, this lack of a global institutional unity is a sign that the Orthodox would lack this mark of being "One". For the Orthodox, as I said, unity resides in having one faith, not one institution. For the Orthodox the plethora of philosophies, of forms of the expression of faith in the RC is a sign that there is not "one faith", but that the RC would have changed the faith along the years, keeping the words and changing the meanings.
So, if you think that unity is primarily institutional and that the institution is what is necessary to preserve the faith, even if changing its outter forms, than you should become a RC. Papal supremacy is just a logical consequence, the Pope being the highest institution inside the institution.
But if you think that the content of the Faith is what constitutes unity, than you allow for much less change in its outter forms, and accept more different institutions with different forms of governing themselves, than you should become an Orthodox.
Institutional unity with multiplicity in form of faith = RC
Unity in form of faith with institutional multiplicity = Orthodox