Magnafluxing or Pressure testing?

Which is the better way to check a head?

Not sure, but I can tell you magnafluxing won't work on an aluminum head. ;)

Magnaflux will find a small crack that hasn't yet penetrated the combustion chamber, or the water jacket, pressure testing will not. You are also more likely to be able to find somebody to do Magnaflux.

On an aluminum head you can use dye-penetrant or eddy-current testing.

Dye-penetrant will work on an iron head too. Problems are you must get it VERY clean first, and it is a messy procedure, but you can do-it-yourself if you feel so motivated!

Pressure testing might still not show cracks the way magnafluxing does. Glass bead cleaning does a GREAT job at highlighting cracks, then magnaflux. B heads are prone to have external cracks that are quite easy to spot once clean.

As Jeff said, glass bead. I've used white sand also cause that's what's in my cabinet. Be sure to blast in the ports and below the seat area. I have one that I did that to and you sure didn't need magnafux to see the crack. Magnafux is pretty cheap where I deal so it's good insurance. Nothing shows up after that, I'd say your pretty darn sure it won't leak.

Does anyone know if soaking an old head in a rust eliminator product will remove enough rust to allow the head to be magnafluxed?

I doubt it. Most shops I know want them blasted, either them doing it or you.