Objection 1. It would seem that backbiting is a graver sin than tale-bearing. For sins of word consist in speaking evil. Now a backbiter speaks of his neighbor things that are evil simply, for such things lead to the loss or depreciation of his good name: whereas a tale-bearer is only intent on saying what is apparently evil, because to wit they are unpleasant to the hearer. Therefore backbiting is a graver sin than tale-bearing.
Objection 2. Further, he that deprives. a man of his good name, deprives him not merely of one friend, but of many, because everyone is minded to scorn the friendship of a person with a bad name. Hence it is reproached against a certain individual [King Josaphat] (2 Chronicles 19:2): "Thou art joined in friendship with them that hate the Lord." But tale-bearing deprives one of only one friend. Therefore backbiting is a graver sin than tale-bearing.
Objection 3. Further, it is written (James 4:11): "He that backbiteth [Douay:,'detracteth'] his brother... detracteth the law," and consequently God the giver of the law. Wherefore the sin of backbiting seems to be a sin against God, which is most grievous, as stated above (20, 3; I-II, 73, 3). On the other hand the sin of tale-bearing is against one's neighbor. Therefore the sin of backbiting is graver than the sin of tale-bearing.
On the contrary, It is written (Sirach 5:17): "An evil mark of disgrace is upon the double-tongued; but to the tale-bearer [Douay: 'whisperer'] hatred, and enmity, and reproach."
I answer that, As stated above (73, 3; I-II, 73, 8), sins against one's neighbor are the more grievous, according as they inflict a greater injury on him: and an injury is so much the greater, according to the greatness of the good which it takes away. Now of all one's external goods a friend takes the first place, since "no man can live without friends," as the Philosopher declares (Ethic. viii, 1). Hence it is written (Sirach 6:15): "Nothing can be compared to a faithful friend." Again, a man's good name whereof backbiting deprives him, is most necessary to him that he may be fitted for friendship. Therefore tale-bearing is a greater sin than backbiting or even reviling, because a friend is better than honor, and to be loved is better than to be honored, according to the Philosopher (Ethic. viii).
Reply to Objection 1. The species and gravity of a sin depend on the end rather than on the material object, wherefore, by reason of its end, tale-bearing is worse than backbiting, although sometimes the backbiter says worse things.
Reply to Objection 2. A good name is a disposition for friendship, and a bad name is a disposition for enmity. But a disposition falls short of the thing for which it disposes. Hence to do anything that leads to a disposition for enmity is a less grievous sin than to do what conduces directly to enmity.
Reply to Objection 3. He that backbites his brother, seems to detract the law, in so far as he despises the precept of love for one's neighbor: while he that strives to sever friendship seems to act more directly against this precept. Hence the latter sin is more specially against God, because "God is charity" (1 John 4:16), and for this reason it is written (Proverbs 6:16): "Six things there are, which the Lord hateth, and the seventh His soul detesteth," and the seventh is "he (Proverbs 6:19) that soweth discord among brethren."
Source: Summa Theologica (New Advent)