I have to vote for "Totally subjective and unrelated to actual statistics." One ride might feel hilly one day and not so bad the next. If it FEELS hilly, darn it, it WAS! That said, the King's Tour of the Quabbin, with 7,000 feet of (extremely short, steep, rolling hill-type) climbing in 100 miles, qualified as Definitely Hilly in my book. I compare all other rides to that and determine hilliness from that completely non-empirical scale.

Plus, I usually just look at total ascent and maybe steepest climb, so I can't really choose one of the options because I don't know how my rides fall on that scale.