This depends on a number of factors:
1.The amount of traffic that the floor will have to cope with:
Clearly if a floor is going to be in a very high traffic area, the likelihood that it will need a light sand and refinish to keep it looking perfect is greater than a floor that has very little traffic and is less likely to need refurbishing. One can therefore argue that a thicker wear layer may enhance longevity.
2.The type of finish:
Some finishes are only achievable in a factory and are either very hard or even impossible to replicate in situ. In these cases, sanding the floor will remove everything that you have bought the floor for, as a sanding will strip the surface back to bare wood. Clearly, beyond any additional durability afforded by a thicker wear layer, an argument can be made that a thinner wear layer may well suffice.
That said, for a finish that is readily replicable in situ, such as a simple clear or coloured oil or clear lacquer, then having the extra material afforded by a thicker wear layer will allow more frequent or more aggressive sanding should it be needed.
Additionally, some finishes are harder and more wear resistant than others and are therefore less prone to needing refurbishment.
3.The construction of the substrate:
Some substrates are more dense and robust than others, for example HDF (High Density Fibreboard) is denser than Spruce. The significance of this is that a thinner wear layer on a more robust substrate (e.g. 2.5mm on HDF) may perform better than a 3mm wear layer on a softer core).
To conclude:
A low traffic floor in a domestic setting will often find that a 3mm or 4mm wear layer with a softwood core will be more than adequate, as will a 2.5mm floor on an HDF core.
Higher traffic areas will probably want to move up to a 4mm to 6mm wear layer with a more robust core (possibly Plywood) but taking into account the finish.
If you would like any advice, please call Upton Wood Flooring on 01491 628765