The choice depends primarily on the speed and accuracy required: Similarly, there are many ways to calculate the delay of a wire.
In order to increase accuracy (and decrease speed), the most common methods are: Often, it makes sense to combine the calculation of a gate and all the wires connected to its output.
The idea behind the semi-custom design method is to use blocks of pre-built and tested components to build something larger, say, a chip.
As chip dimensions get smaller, the delays of both gates and wires may need to be treated as statistical estimates instead of deterministic quantities.
In both cases, it is critical to capture the dependence on the underlying variables, such as threshold voltage and metal thickness, since these result in correlations among the delays of nearby components.