The only difference would be if your pre-amp has XLR outputs. If the preamp only has unbalanced outputs (i.e., RCA jacks), then the XLR will do no good.
Also - I believe the MC7100 was not initially designed as a fully balanced unit internally. You will not get the full benefit of XLR unless both pre-amp and power amp are designed as fully balanced internally.
Having said that - if you have a long run from the pre-amp to the power amp, the RCA connections may be prone to hum & noise. The XLR (balanced) connections should clear that problem up.