Network Monitoring: Bits v Bytes

[chrisptech]

This has probably been done to death before but any time I try to search for an answer it comes it short.

We run a network with 15 switches and around 30 different servers. I want to monitor the network and I don't know if I should monitor for bits or bytes? I understand that bits measures speeds and bytes measures data, but I am struggling to decide which is more important? Can I tell if a link is getting overloaded by the bits speed or bytes data?

Maybe I'm over thinking this