X-Cut when factory filed is faster than STIHL chain. X-Cut is "race chain" for the masses and well-thought. It holds an edge quite well and is quality chain. It is incredibly smooth cutting, and however one analyzes it is great looking, great performing chain.
STIHL has highest quality chain I've seen. It has a high chrome and nickle content giving it great wear resistance (to filing also), resistance to rust, and has the optimum balance of speed to longevity so is "the most productive chain for general use." I understand that's why STIHL bars & chain are commonly seen on Husqvarna saws. --To that point though, Husquvarna brought X-Cut in-house (Oregon was making it), and STIHL bar prices are getting pretty spendy, so I'm seeing fewer STIHL bars on Husquvarna saws and more X-Cut chain being used, including on STIHL saws.
Mind you "what's best" depends on your preferences, I run different brands of chain depending on the use and am not saying one is better than another in all cases.