Mu-metal or amorphous core step up transformer?

can you describe your experiences with the differences between mu-metal and amorphous core SUT? What do you prefer and why?
Some companies like Lundahl offer their transformer with both kind of materials for the core.
Some prefer mu-metal, some amorphous core.
Thanks for any comment.
You might also try posting this on the K&K forum on the Audioasylum site. Kevin Carter the US distributor for Lundahl has great ears and lots of experience with these products.