4 channel scope vs 2 channel scope
The times when I have needed a four channel scope, are few, but when I do there is no replacement for it.  You mentioned serial protocols, using four channels can really help debug  SPI communications, especially if the scope is fitted with a analyser to convert the SPI waveform into actual characters!

Having said that, buying a two channel scope, and then hiring a high end four channel with analysers (fft/spi and so on) might be a worthy alternative.

The saving you make up front, can go towards the hire cost of when you actually need it!  Might be worth investigating the hire cost upfront so you know what you might be in for.