I have pretty much a 55/45 split between those liking 18 and 15, which puts me in a bit of a difficult position. I liked Size 4 given that it was between 15 and 18, but since I'm forced to choose I'll take slightly larger text over slightly smaller text. If that changes then I will reevaluate, but I'm seeing more positives about 18; it makes it easier to read on mobile even if it involves a bit more scrolling.