Pi should not be the first choice unless you just wanna dip your toe in the water with limited investment or you are real experienced in the trade. While the hardware is powerful enough for many use cases, it is very limited in external connectivity which really hampers its potential as a NAS/multi-purpose server.
CPUs often get less efficient (in the sense of work done per watt) when they are pushed to their limits. Unless you are running the server at full load all the time, the power consumption of a typical x86 system is quite manageable (~30-50W) at idle to low usage. Newer hardware is surely more efficient as newer designs are relatively faster and often have more power conservation technologies built-in.