Not better sound, but louder (just a tad). The sub will sum both inputs, which doubles the voltage and results in a +6 dB increase in the level. The same thing can be accomplished by turning the volume knob up a smidge.
You can use either the left or right input or use a Y as you say. Most sub manuals will say to use the left input if you connect only one. Some subs designate a specific input to use if you want to disable the sub's internal crossover and you want to use that one if you connect from the receiver's subwoofer pre-out so that the receiver will control the xover frequency.
