Short answer: Without more context, it’s impossible to say what you mean by “which is better.” The phrase “which is better” is a general comparison prompt, while “which one is better” usually refers to a specific set of options. If you share what two (or more) items you’re comparing and what matters most to you (cost, quality, features, user experience, etc.), a precise recommendation can be made. If you’re asking about language usage:

• “Which is better?” asks for a general comparison between two options.
• “Which one is better?” typically implies you’re choosing from a defined set of items.

If you have a concrete comparison in mind (for example, two smartphones, two diets, or two software tools), list the items and the criteria you care about, and the best choice can be evaluated directly.