I haven't thrown the Saga, but maybe this information could help:
Since the Saga uses the same coverstock that is used on the Rising Star, Shooting Star, Shout and the Scream, Ultimate Vision, I'd assume there was a rather large difference between the Defiant Soul and the Saga. (Add in the DS's asymmetric core)

Saga: http://www.123bowl.com/ball.cfm?ballid=2893

The saga was asymmetric as well with and intermediate diff if around .026 or so. So they may be close. I loved the saga as well

How can they be close if the Saga used a coverstock that's not on Roto-Grip's weakest balls?

Oil changes as covers get stronger. I'm not saying it will react the same side by side but on today's oil they should have a very similar motion

Because of the core, and if they're drilled the same, the motion will be similar, but I'd still believe the Defiant Soul should handle and require more oil than the Saga with the same surface. The nature of house blended and wet/dry patterns will make many balls behave the similarly; it often takes a true sport pattern to expose the true differences between balls.