I wouldn't go so far as to say they were more tender, they deinitely cook more even. We all know the bottom when laying flat on the rack cooks faster than the top that's a given, on the standup rack both sides cook about the same pace. Those ribs were very lean (bought them on a trip out of town) along with some suasage links. The outfit where I bought them raises their own hogs at an adjacent farm transports them to the store and processes them onsite, it's the leanest pork I've ever encountered. SOOO, of all the ribs I cooked out of this batch they were the juiceiest, the 1 1/2 less cooking time is a plus.
I'm cooking the last ribs of the batch Tues, so I'll try the racks again then we'll know the definitive answer. The link suasage I bought there smoked with apple is the best I have ever had bar none, flavorful, tender and not a lot of grease rendering out of it.