Regardless of the cooking temperature, back ribs are quicker and easier to prepare than spares, especially for those who don't have something like the WSM to cook them on. Also, with spares you usually have to either trim them or deal with all the cartilage. The resulting popularity of loin backs is probably why they are more expensive. Certainly not because they are "better".
All ribs are good when well prepared, but spares are meatier and cheaper. That's worth a little extra time.