I found that there are two formulas that can be used to calculate transmission delay. The first is datasize/bandwidth, and the second is datasize/transmission_rate. So which one is correct? Or why are there two formulas to calculate transmission delay?
I'd like to figure out the correct formula for calculating transmission delay.
In the computing meaning, "bandwidth" is used interchangeably with "data rate". Both are given dimensionally in terms of units of bits or bytes per second, and represent the same quantity, so the two formulae you have actually say the same thing. They give the transmission delay - i.e. the time it takes to push that many bytes onto the link. Add the propagation delay and you have your total end-to-end delay to send this data.
However, it's worth noting that if you speak to an electrical or RF engineer about a link design, "bandwidth" will likely be measured in Hz and mean something different from "data rate". However, in that case, you would expect a different, more complex formula for transmission delay vs bandwidth, which includes spectral efficiency and various error correction overheads, and is different from the formula you presented here. The same likewise holds when speaking in the context of different layers, just with the right corrections for the assumptions being made (e.g. when discussing network engineering, you have bandwidth of a physical link, and you account for a different set of overheads)