I'm not gonna say which one one I voted for (mainly because I don't want to be stoned to death), but having tried both drinks I will point out the differences I noticed between each.
Well, for starters. I found that the foam for Butterbeer tends to stay seperate from the actual drink, whereas the foam for Lefou's actually mixes into the drink as you drink and it actually adds to the flavor as well (Can't figure out why some say to do away with the foam.....I actually quite like it).
I find Lefou's to be less.....soda-ey than Butterbeer. Lefou's is way sweeter, fizzier, more tart, and way more refreshing.
Lefou's is a much thicker concoction. You cannot chug it, as I found out the hard way.
Lefou's tastes better the more you drink it, taste of the drink kind of changes the more you drink probably due to the foam slowly mixing into the drink as you drink.
Also: Lefou's doesn't make any mistake telling you what it is. The drink is frozen apple juice and they do state what flavor the foam is, though I can't remember it at the moment
Butterbeer and Lefou's are both pretty different drinks with vastly different flavors. I'd say both are about equal, but if I had to choose one....well, once again, my fear of stoning won't allow me to divulge my choice (though I'm pretty sure y'all can guess what it is)