the free motion feet that I have are the one on the right and the one on the left in Tammi/ArchaicArcane's photo.
The plastic/lucite or whatever, disappears from my sight when using these feet my total view is where the needle will enter the fabric next, fortunately my 301A does not mind 'going slow' . For straight lines and ITD the regular foot works fine for me but my DD likes to use the zipper foot ( the one where you can switch sides ) so bottom line is everyone needs to find what works best for them and has least shoulder strain, IMHO.
The visibility afforded by the slant needle is an awesome plus, I am whispering so my BERNINA does not hear, she is such a Diva.