Is it better to loosen B230 head bolts with a breaker bar slowly or use an impact wrench? Which reduces the risk of the bolts breaking off?

I used a pneumatic impact with no problems. I'd use it again if I had to do another job like it.

Breaker bar. Don't even think of an impact wrench for something like this, it is far too critical that you know what is going on as you loosen and tighten these bolts.

You can bust up a socket on these, I did 3 years ago on mine, but believe everyone that has posted here: use a breaker bar. It really does not take that much strength.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

Breaker bar.
It's the reason I do my lifting.
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84 242Ti IPD bars&springs, 89 745 16v M46 IPD bars, 89 744 16v M46 IPD bars, 90 745 AW70, 91 245SE AW70 IPD bars, 93 245 CLassic M47

I vote for the breaker bar. It gives you a much better feel for what's going on. If one of the bolts is unusually tight, you will notice it before it's too late. However, it is more common to break head bolts when torquing them down than when removing them.

I prefer a 24" bar (as opposed to an 18") for the extra leverage.

In trade school, we use breaker bar.

Impact gun. You can develop way more torque with a breaker bar and a cheater bar ontop of it. I have seen instances where people have sheared stuff off using a breaker bar that they would not have sheared while using an impact. I have always used and impact and in all of my years I have only had one bolt shear and that was going back together.

Mark