I don't think we need to focus on power blows vs break blows on themselves, but rather how they influence the overall gameplay mechanics. Let's compare them.
- Power blows could only be done after your health drops below 50%.
- Break blows can only be done with full meter.
This basically confirms the power blow as a sort of comeback mechanic. Not only that, the Power Blow is limited to that specific round since it's based on your health. Since meter carries across rounds, Break blows are more versatile in the sense that you can even use it at the beginning of the next round. And more meter is given to the better player.
- Power blows are guaranteed only after a critical burst. For a critical burst, extending stuns is generally required.
- Break blows are guaranteed if your opponent has less than 50% meter and are in a stun that lasts longer than the start-up frames of the break blow. (It can also be argued to be "guaranteed" if you successfully use it is as a Sabaki, but that's a grey area that I'm not going to touch at this moment.)
Since there is no slow escaping in DOA6, in specific situations a single good hit can guarantee a break blow. It requires awareness of the situation rather than playing the guessing game against your opponent.
- A blocked power blow causes a guard break with significant frame advantage that allows a guaranteed follow-up.
- A blocked break blow causes an insignificant guard break that gives you nothing, while losing all your meter and thus requiring meter buildup to use it again.
There is nothing to defend against power blows specifically.
Break blows can be defended with break holds, which require meter.
I put these points together because, wasting your meter with a failed break blow is a lot more costly than failing your power blow. Having no meter equates to not being able to defend against a power blow (and also fatal rush).
- Power blows could be redirected to take specific advantages from within the environment.
- Break blows hit in the direction you're standing.
Probably not really that important in most cases, but it does influence positioning a bit, where you could more easily ledge an opponent with the power blow compared to the break blow.
- After the power blow lands, there is nothing other than simply letting the animation play out. An alternative is the power launcher, which needs to be done instead of the power blow, and gives you a very high launch height instead.
- The break blow allows you to cancel it after it lands so you can do whatever launch and juggle that you want.
This again gives more flexibility to the break blow.
After listing this, I think the break blows are objectively better.
It's not a comeback mechanic, meaning, the one that is ahead is rewarded, rather than the one behind.
The stun guessing game has mostly been taken out of the equation with the break blows.
There is a high risk after using it, especially compared to the power blow. Even after it lands, you are for a short time left completely defenseless against fatal rush/break blows (if stunned) because your meter goes to zero. It requires the attacking player to keep playing well.
Break blows can be used in different ways at different times. For some quick damage after a single stun, a way to extend juggles, sabaki...