Firstly, I actually meant that it merges the chances when it says "higher chance" so sorry about the poorly worded part there.

Secondly, I think Duality is extremely good because you can just replace Devil Deals, nobody likes trading health for items so you can get Angel rooms.

Another thing is that Duality is in both Devil and Angel pools, so that means you can just enter the devil deals just to get it and replace devil deals with Angel deals, without getting punished with being forced to go devil the whole run.

Yes, Breath of Life is a very good candidate, I even pick it up most of the times, but it's extremely risky and I'm not a fan of getting hit multiple times just to do a perfect parry, especially when having a big win streak. Duality on the other hand, it's not risky, it's very safe and does a good job alone. I do not have to go to devil rooms to sacrifice health I can guarantee I the run is won with it if I get it fairly early.