What type of cutting disc should I use for cutting steel rods?
Depends on you, as much as anything.
First off, don't forget eye protection and to work in a location where the sparks flung won't set your house on fire (sometimes hours later, if they get off into a nice cozy bed of lint or shavings in a corner.)
All cutoff discs are consumable, in the sense that they either break or wear down too small to be of use, so buy them in multi-packs.
Very thin discs are easier to break, but cut faster since they remove less metal. If you have a steady hand so you don't break them so much, they work well.
Thicker &quot;reinforced&quot; discs cut slower, since they remove more metal, but are somewhat harder (though not impossible) to break.