First, I don't think I've been led astray on the Delta L fare issue. Thanks to a 1998 switch I'm both Delta Gold Medallion and AA Platinum AAdvantage, most likely losing Delta elite this year but earning AA Exec Platinum, and speak from LOTS of personal experience with both. Nobody has fed me a line on either airline. My experience confirms that Delta's premium for upgradeability vs. the cheapest available fare is typically 2-3 times more than the cost of purchased AA upgrades for that route. This, to me, more than offsets AA's relative stinginess with free upgrades.

Second, on balance, the smaller the increment the less likely you are to waste miles. For BOS-ORD, 867 miles, I have to use 1,000 miles of AA upgrades but would need 1,600 miles of Delta's if Delta flew that route. BOS-SFO is 3,000 miles of AA upgrades, 3,200 miles of Delta's (again, if Delta flew that route). There are examples where any given size works out best, and one person's travel patterns may happen to use those routes a lot, but on balance the smaller size will statistically waste fewer miles overall.