I haven't seen a Rumble between the two of them posted and I've been curious for a while over how the two of them would match up since both are similarly meta level swordsmen.
For the purposes of this rumble, Kenshin's sakabatou has the same level of durability as a nichirin sword.
Tokyo-arc Kenshin vs Pre-Mugen Train Tanjiro
Kyoto Arc Kenshin(Pre-Hiko training) vs Pre-Hashira arc Tanjiro
Kyoto Arc Kenshin (*Post-Hiko training) vs Post-Akaza arc Tanjiro
No Demon Tanjiro.
*Since Kenshin never actually becomes stronger following the end of the Kyoto arc, Jinchu and Hokkaido arc Kenshin are essentially treated as the same.