I used to drink large amounts of both, with no preference toward either.
Now, in Pepsi over here, some sugar has been replaced with artificial sweeteners, and that is a no-go for me. I'm talking about the regular Pepsi, not any diet soda or "Pepsi MAX".
Coca Cola here is still all sugar, but they sell their Zero more, and some flavour varieties are only available with artificial sweeteners.
I don't know if I'm imagining it or not, but it feels to me like the regular one tastes a little different now: as if the flavour had been adjusted to be closer to the one with artificial sweeteners.
I had even sent some cans back for a refund after that happened.
My favourite has instead become a cheaper domestic variety that taste more like "cola bottle candy".
Thankfully, no high-fructose corn syrup over here.