Remind me -- does it come only in one size or the other?
On my sport band, I can wear either the S/M or M/L straps, so I chose to wear the S/M size which leaves less overhang (or "underhang", as it were).
And +1 to what Relentless Power said -- the larger size will also have more leeway in case it slips (which shouldn't be a problem anyway, but just that extra assurance would be nice).