If you connect FX2 to line input then the signal goes from analog to digital to analog (in the FX2), then analog to digital (in the Presonus).
If you connect FX2 to SPDIF the the signal goes from analog to digital (in the FX2), then digital into the Presonus. Fewer analog/digital conversions is better so SPDIF is better.
I agree with the above - multiple conversions would be pretty pointless in this instance.
However, digital cables tend to be much more fragile than analogue cables and decent quality S/PDIF cables can be pretty costly. Cheap digital cables can often bring annoying data glitches into the system too, which can be pretty time consuming!
If you do own good quality S/PDIF leads though, go with this option and simply make sure your sample rates are matched
You will.also have to run the session in your daw at the same.sample rate that you are running the axefx. If the axefx is running 48k, the daw has to run 48k.