That's a matter of personal preference. Since your fronts are "large" (as they must be for you to have the sub options of LFE or sub+L/R), you need to decide if you want for the sub to get the LFE channel only (the "LFE Only" choice) or if you want a "double bass" scenario in which both the fronts and the sub get low frequency data from the front channels (the "Sub+L/R" choice).