It would be physically impossible for a human to choose between Vanilla and Chocolate here is why:
It depends on many factors such as Combination, and strongness.
Combinations: In my opinion, this is the most important factor, to be able to decide between chocolate and vanilla. For example: if you combine vanilla ice cream with chocolate brownies, it will be amazing, but if you combine chocolate ice cream with a chocolate brownie, it will be a disaster (just my opinion ^^). So , i think the combination factor is very important on deciding who wins. In my opinion, vanilla ice cream has more things that will make magic if combined with, for example: cake, brownies, cookies, cupcakes, waffles, pancakes, and more. But i certainly can't think of anything that goes very well with chocolate ice cream, other than cake. So, in my opinion vanilla has won this round, (you can't deny it
)
Strongness: This won't make much sense, but the strongness of the chocolate/vanilla affects A LOT, it's flavor. If you have EVER seen 90% chocolate on the market, (just getting the word out) DO NOT TOUCH IT, IT'S EXTREMELY BITTER AND DISGUSTING, cough cough. So, most dark dark dark and pure chocolate will most likely have less sugar and taste more horrible, but in my experience of eating ice cream, i have tried 80% pure vanilla ice cream and it is not half bad. So, pure chocolate = ew, and pure vanilla = meh.
In conclusion, VANILLA FOR THE WIN! And that was my explanation why. ^^