We can decided which method of deprecation is best on the basis of usefulness (benefits) provided by the asset.

When we prefer SLM:-

we preferred SLM where the equal amount of benefit is derived from the asset through the useful life of the asset. e.g A room's capacity of storing the goods remain same in each year weather old or new.

When we prefer DBM:-

we preferred DBM where resulting benefit from the asset diminishing (decreasing) year by year.