The sages regarded Abraham as a greater tzaddik: where No’ach walked with God, Abraham walked before God (Gen. 24:40).
There is an analogy: as a young child walks with its parent, but an older child is trusted enough to walk a little ahead of the parent, so No’ach was not yet as morally mature as Abraham was.
There is another comparison, according to Rabbi Levi Yitzchak of Berditchev.
One type of tzaddik is devoted to God and to human beings, but another type is devoted to God and not so much to humans.
The second type is No’ach, who for all his piety is too ready to give up on other people.
The first type is Abraham, who never ceases to endeavour to bring his contemporaries to God. Abraham and his wife Sarah dedicated their lives to what today is called outreach; No’ach gave up and settled for safety in the ark and the hope that a new day would dawn.
Both types are alive and well in our own day; some say, “Let me at least preserve my own and my family’s religious standards,” whereas others keep on trying to bring their contemporaries under the proverbial wings of the Divine Presence.
Rabbi Levi Yitzchak, if he were alive today, might suggest that there are two types of the Abraham model.
Some follow an outreach method which comes on rather too heavy, being a little too superior and patronising. Others work more subtly, by example rather than what looks like propaganda. Both seek the same goal, but one is more gentle and constructive.
Baruch HaShem, both bring new strength to Judaism, but there are valid ethical questions about how best to pursue the goal.