Watch out - this isn’t correct and a common mistake
Standard blank rotors are grey cast iron (G250 or G3000 are the industry standard materials) and usually just fine for most street and even “trackday” cars. These cast irons have carbon content around 3.25%
“High Carbon” rotors are not stronger (if anything they have a lower tensile strength). There’s no industry material standard (exact alloy is proprietary for most companies) but typically have a higher carbon content around 3.65%
The additional carbon forms tiny nodules that increase thermal conductivity which makes them more resistant to thermal cracking and warping, and also absorb vibration, which makes them less likely to squeal. Which is why most OEMs have switched over to it.
Brake rotors despite general impressions are not typically under much mechanical stress - thermal stresses and heat dissipation and heat capacity (basically mass) are a much more important consideration.
Short answer, either will work. Centric 125 rotors usually have partial anti corrosion coating on hubs and vanes like Centric 121 “premium” rotors, while the 120 are usually bare. Any of them will be fine even for cars that see track use and track pads.
Steel rotors do exist. Solid steel rotors are lightweight brakes, usually only for drag cars because they’ll easily warp if you put much heat into them.
Also, some fancy aftermarket rotors are laminated steel to allow lighter rotors to be used. Despite the picture in the article below, even pretty drilled/slotted aftermarket rotors are not steel. This is usually reserved for only very high end two-piece rotors used in motorsport. (even most two piece use cast iron)
Here’s a good article if you’re interested
https://trade.mechanic.com.au/news/disc-rotor-materials-g3000-g250-high-carbon-what-the-flux