I bought sihoo m57 earlier this year due to wfh. It was comfortable.
Now joined new company, the gave steelcase think. Even with company discount, it is still 1.7k vs 8xx for the m57.

My opinion is m57 is more comfortable and worth to buy.

Pros m57
1. Mesh all around. Room doesn't have AC. So this is good
2. Able to add head rest (not too expensive)
3. Adjustable and lockable lean position
4. Adjustable lean tension
5. Relatively cheap

Cons:
1. Arm rest less adjustable (3D. But not a deal breaker)

Steelcase think pros
1. Backrest is mesh, while the seat is foam. A bit warmer feeling
2. Armrest is 4D adjustbale
3. Got bouncy feel when you sit on it

Cons
1. Headrest is 500+. Cannot afford lah.
2. Only not all lean angle lockable
3. Tension only 2 level adjustable (not to different)
4. Expensive. If own money, definitely will not buy