The price for genuine Bernina feet is higher than most, but they're ALL metal construction and last forever. I bite the bullet and don't buy generic for my Bernina. I've had generic feet in the past on my Janome and they usually are junk.
I have the #57 foot, but if I had to do it again, I would try both before buying. I was used to the guided foot so bought that, but I think I'd like the unguided foot too.