Wool vs. cotton for your tallit katan

The Torah has several laws about beged, cloth. The most specific is about tsara'at, which affects cloth of wool or linen:

וְהַבֶּגֶד כִּי יִהְיֶה בוֹ נֶגַע צָרָעַת בְּבֶגֶד צֶמֶר אוֹ בְּבֶגֶד פִּשְׁתִּים: (ויקרא יג, מז)

The Talmud on Shabbat 26b presents a binyan av from that verse to teach that all mentions of beged in the Torah refer specifically to wool and linen. This applies to three other verses about tum'ah on a beged, and another verse saying that tsitsit goes on a beged. They all mean a wool or linen beged.

Everyone agrees to this binyan av. But the halakhot in these verses can still affect other materials, like cotton, when ribbuyim come into play. The question of which materials of which sizes are mekabbel which types of tum'ah is a huge debate on 26a–27b. Here are some charts (PDF). There's an even better chart in the footnotes of Mesivta.

The most practical example for nowadays is that of tsitsit. Is cotton acceptible for the beged of a tallit kattan? Or should you insist on wool?

The Torah says tsitsit belongs on a "beged." Rav Nahman on 27a–b says that all materials except wool and linen are excluded from the mitsvah of tsitsit. Rava disagrees on 27b, maintaining that all materials are indeed included in the mitsvah.

Their dispute continues into the rishonim. The Rif and Rambam both pasken that only wool and linen require tsitsit mide-orayta, and other materials require tsitsit mide-rabbanan. In the words of Hilkhot Tsitsit 3:2:

הלכה א – כסות שחייב אדם לעשות בה ציצית מן התורה היא כסות שיש לה ארבע כנפים או יותר על ארבע, ותהיה מדתה כדי שיתכסה בה ראשו ורובו של קטן המתהלך לבדו בשוק ואינו צריך אחר לשומרו ולילך עמו, ותהיה הכסות של צמר או של פשתן בלבד.

הלכה ב – אבל טלית של שאר מינין כגון בגדי משי ובגדי צמר גפן ובגדי צמר גמלים וצמר ארנבים ונוצה של עזים וכיוצא בהן אין חייבין במצות ציצית אלא מדברי חכמים כדי להזהר במצות ציצית, והוא שתהיה מרובעת או יותר על ארבע ויהיה שיעורה כשיעור שאמרנו, שכל הבגדים האמורים בתורה סתם אינם אלא צמר ופשתים בלבד.

Hasagot ha-Ra'avad disagrees with 3:2:

כתב הראב"ד ז"ל כמדומה אני שזה הוא כלך ולא מחוור עכ"ל

Also against the Rif and Rambam: Tosafot on Menahot 39b, the Rosh, Semag, and Mordekhai all pasken like Rava that materials other than wool and linen also require tsitsit mide-orayta.

The dispute carries on between the Shulkhan Arukh and Rema (O"H 9:1), the former siding with the Rambam:

אין חייב בציצית מן התורה, אלא בגד פשתים או של צמר רחלים, אבל בגדי שאר מינים אין חייבים בציצית אלא מדרבנן.

הגה: וי"א דכולהו חייבין מדאורייתא, והכי הלכתא.

And so you find many Ashkenazim wearing a cotton beged for the tallit katan, following the Rema. But the Mishnah Berurah has this to say on the Rema's pesak (se'if katan 5):

והכי הלכתא - וירא שמים יחמיר על עצמו לחוש לדעה הראשונה ויעשה טלית של צמר כדי שיתחייב בציצית מן התורה לכו"ע. ונכון שיעשה בין הט"ג ובין הט"ק של צמר:

And so you find that in yeshivish communities, they're all wearing wool. Other posekim who prefer wool are found in footnote 8 of this essay by Rabbi Ari Enkin.

Rav Avigdor Nebenzahl shlit"a notes, in be-Yitshak Yikkare on this Mishnah Berurah, the testimonies that both the Vilna Gaon and the Hatam Sofer did not use wool for their tallit katan. I wouldn't doubt the yir'at shamayim of either of them. The Hatam Sofer explained that since he found wool uncomfortable, it didn't count as a beged for him.

Rav Nebenzahl shares responses from Rav Shlomo Zalman Auerbach to each of these testimonies: The Vilna Gaon wore a tallit gadol made of wool all day long anyway; and contrary to the Hatam Sofer, wearing a beged for the purpose of a mitsvah is enough reason to call it a beged. So the rest of us who want to be yir'ei shamayim should be mahmir and wear wool.

Nonetheless, those wool-wearers who would consider themselves more frum than the cotton-wearers should know that there are some big gedolim in the latter group. But I'm sure there aren't any wool-wearers like that.

Personally, I have no preference comfort-wise between wool and cotton, so I'm happy to be counted among the Mishnah Berurah's mahmirim. Wool costs a bit more. But remember the story about the Vilna Gaon weeping over his tsitsit while on his deathbed: "For just a few pennies in this world you can attain the World to Come!"