It depends. If I want a quick one I use the right. If I'm going for an all nighter I use the left. If I'm PMSing (I get extremely horny) I have to use both.
whichever one feels the best...actually i take turns so they don't get jealous...the last thing you want is a hand fight while trying to get off...if my feet could reach my cock i would use them too...oh well...