Comparing the relative strength of two swords in the "One Piece" series can be subjective and dependent on various factors. Both Enma and Shusui are significant swords with unique characteristics and histories. Here's a comparison of the two swords:

1. Enma: Enma is one of the 21 Great Grade swords recognized as Meito in the series. It was initially owned by the legendary samurai, Kozuki Oden. Enma is known for its tremendous cutting power and ability to absorb and channel Haki. Its immense power can be difficult to control, as it requires tremendous strength and mastery to wield without being overwhelmed. Enma has proven to be a formidable weapon in Zoro's hands.
2. Shusui: Shusui is also one of the 21 Great Grade swords and was previously owned by Ryuma, a legendary samurai of Wano. Shusui is renowned for its incredible sharpness and durability. It is a large and powerful blade that has become one of Zoro's signature swords. With its exceptional cutting ability and strength, Shusui has aided Zoro in numerous battles.

In terms of pure strength and cutting power, both Enma and Shusui are formidable swords. However, it is challenging to definitively state which sword is stronger as their strengths and unique characteristics may suit different situations and fighting styles. Additionally, the individual skill and abilities of the wielder, in this case, Roronoa Zoro, also play a significant role in determining the effectiveness of a sword.

It's important to note that "One Piece" is an ongoing series, and new story developments may have occurred beyond my knowledge cutoff in September 2021. To get the most up-to-date and accurate information on the swords' relative strengths, it is recommended to continue following the manga or anime series.