The "Forsaken Mistress" was available to order in various versions about this time last year. You can see the offer
here.
It's a "forsaken" mistress because back when they released the Fusion Battle Mistress, it was a design they considered and rejected. Or forsook. You can see the original FBM offer page
here. There have been various other incarnations of the Mistress (or Battle Mistress) line both before and after the FBM-- Straight Handled, Ergo, Bushmaster, Nuclear Meltdown Fusion, etc.
A Battle Grade version of the FBM (made of SR101 instead of INFI) was released a year or so ago and is still available to order from the Busse website. See
here.
I think most people would agree that INFI is more desirable that SR101, but SR101 is a lot more affordable and more readily available. Putting the metal preference aside, I think which Mistress is more desirable is a matter of personal preference.
My favorite is the Straight Handled Battle Mistress (SHBM), which was the original Mistress and the first model offered in INFI, but those are quite expensive now and can be pretty hard to find. I think from a collector's standpoint, one of the rarer versions of the SHBM would probably be considered the most desirable.