I have a Bernina 350. I got a #37 (37D) Patchwork foot and also the Patchwork Seam Guide -- seriously worth every penny! Here's what it looks like:
http://www.bernina.com/en-US/Product...ork-seam-guide
It screws into my machine bed to make a hard edge along which to feed my quilt for a consistent seam (can set at 1/4" or up to 7/8" depending on how you set it up). Plus, the seam guide is only like $19 which is amazingly cheap for Bernina.
The Walking Foot #50 comes with 3 different sole plates so you don't need to choose ahead of time. My personal preference is the open-toed foot so I can see my ditch. I don't have good luck with the one with the center guide in it, but I use very thick cotton batting & my Bernina dealer said that may be the root of the problem.
On my Singer I buy almost all 3rd party parts. For my Bernina, as much as I nearly stop breathing when I see the prices (especially on the walking foot), genuine Bernina parts actually do seem to make a difference.