No experience with the two subs, BUT the 30Hz vs 20Hz might make a big difference if your main speakers go to 35Hz or below, since it's very important to avoid excessive sub /mains overlap to prevent the sound becoming very boomy. If you main speakers rolloff significantly above 30Hz then I doubt the 8Hz lower rolloff of the REL will make any difference at all.
For example I have spica angelus mains that go to about 35Hz (but about 6dB down at this point) and if I set the crossover on my rel strata above 27Hz the bass gets very boomy.
For example I have spica angelus mains that go to about 35Hz (but about 6dB down at this point) and if I set the crossover on my rel strata above 27Hz the bass gets very boomy.