* Fearr: This is the most common and general translation for "better," often used for comparing two things. For example, "This is better than that" would be "Is fearr seo na sin."
* Nas fheàrr: This translates to "better" or "rather" and implies a preference. It's often used when choosing between options. For example, "I'd rather have tea" would be "B' fheàrr leam tì."
The choice between *fearr* and *nas fheàrr* hinges on the grammatical structure of the sentence. *Nas fheàrr* is used with comparative constructions.
Therefore, without knowing the sentence you intend to use it in, fearr is a good starting point, but nas fheàrr might be more appropriate in many situations.