What are the advantages of a wider bus in comparison to those of a higher clock frequency?
The only advantage to a wider, slower bus is slightly simpler interface electronics. When receiving data, for instance, a parallel bus requires n gates or flip-flops to accept the data. A serial bus handling the same data will require (typically) an n-bit shift register and then another n flip-flops as holding registers. Plus, the serial bus will require a synchronization process to inform the receiving unit when it has accepted n bits. Much the same considerations apply when transmitting data.

On the other hand, the parallel bus will require n bus drivers, which typically take more silicon and power than internal logic, so a serial bus will probably take less power. The parallel bus will, of course, require a physically bigger cable (more conductors) and a bigger connector footprint, which may be an issue on a crowded board. With only a single signal path being used for the serial bus, differential signalling becomes quite reasonable, which both reduces EMI and increases speed.