Fire seems to be the way to go. Water pokemon are a dime a dozen, and grass pokemon are way more common than fire. So to have greater type coverage earlier, it seems to me fire is the way to go everything else being equal.
I don't know, I don't really have a preference. I just pick whichever starter looks good to me. It just so happens that I chose a water starter three times, a grass starter twice, and I'm going to choose a fire starter for the first time in gen 6.