For attic runs, the better choice depends less on “which cable is better overall” and more on what conditions the attic actually has and what your local code allows. In many homes, NM cable is perfectly acceptable in an attic if it is protected from physical damage, kept within the temperature limits of the cable, and installed correctly. That said, MC cable gives you a tougher outer jacket and better physical protection, which can be useful if the attic is cramped, likely to be walked through, or has a higher risk of abrasion from framing, storage, or future work.
If the attic is unfinished and the cable will mostly lie across joists or be stapled along framing members, NM is often the more common and cost-effective option. It is easier to pull, easier to terminate, and usually cheaper. A lot of residential work is done with NM for exactly that reason. But you need to pay attention to derating and insulation contact rules if the attic gets very hot or if the cable is buried under insulation. High attic temperatures can reduce ampacity, and depending on the setup, you may need to account for that when sizing conductors. That matters more than the brand name on the jacket.
MC is often the better answer when there is any doubt about physical protection. If the attic is used for storage, has service equipment, or has repeated access for HVAC, security, or low-voltage work, MC can hold up better over time. It is also useful where you want a more rugged installation or where local rules are stricter about exposed cabling. The downside is cost, stiffness, and the extra effort required for fittings and terminations. For a simple branch circuit, MC can feel like overkill unless the attic environment justifies it.
One thing I would not ignore is support and routing. Whichever cable you choose, keep it neat, protected from nails and screws, and secured properly. Avoid running it where someone will inevitably step on it. If you have to cross open joists, a bored-hole route or a protected pathway is usually smarter than just draping cable across the tops of framing members.
If you want the short practical answer: use NM if the attic is truly just an unfinished residential space and the cable can be protected properly; choose MC if the cable needs extra mechanical protection or you want a more durable installation. If you’re unsure about attic temperature limits or whether your local inspector prefers one method, it is worth checking before you buy materials.
