katanas are a weapon by design. Sais are a farm tool for planting that are adopted into a fighting style. Sais are only as deadly as their user. If you take the wielder out of the equation, katanas win every time.
Katanas are more capable of damage, and have better reach. Sais (if sharpened) can be used for piercing attacks but are more useful for parrying blows and disarming someone (especially bladed weapons).
You give an unskilled user a pair of sais in combat with multiple opponents, he/she will probably not do as well as if you gave him/her one or two katanas.
Add a proficient wielder into the equation, and then we have a debate. I’d go 50/50. Someone that specializes in their weapon, it could go either way.