Basically, the effectiveness of both depends on what you're healing with the aura. The auras break even at 1322 max hp. Above that hp regen is better. Below that the flat heal aura is better. Technically at max regen the heal aura would be better again but at that point whatever targets are probably regenning so much it doesn't really matter.
So the short of it is, the regen aura is better on brutes/scrappers/tanks, somewhere close to even on most other targets if they have the +hp% accolades, and worse on you and your pets.
Oh, and it's worth noting that the regen from the MM aura is as much regen as a regen scrapper/brute gets from fully enhancing both fast regeneration and integration for healing.
Culex's resistance guide
Pain Domination varies between Corrupters and Masterminds. Their aura skill. Soothing Aura is 'minor healing' over time. Suppress Pain is + to health regeneration(base something like 200%).
As a mastermind I'm kinda curious. How is Soothing Aura? How do the two auras compare to each other?
I'm biased by personal experience-the +regen doesn't seem to help much. But that's likely because the health of myself and my minions is also crappy, so the Regen doesn't go far. And since I have no shielding, there's a lot of damage to go around.