First decide whether you want to reduce limescale in drinking water or throughout the property. Those are different jobs.
For drinking water
Compare filters or RO systems that provide evidence relevant to hardness or the dissolved minerals responsible for scale. Consider the capacity of the system and how much filtered water you actually consume.
For whole-house limescale
A point-of-use drinking-water filter will not protect every appliance. Investigate whole-house treatment separately if that is the objective.
Don't forget the kettle test
If your practical complaint is a heavily scaled kettle, measure the issue before and after treatment using the same usage pattern. That gives you a more meaningful real-world comparison than marketing language.