I have used three different Williams receiver (peep) sights on Marlin 336s: Williams 5D, Williams Foolproof, and WGRS (Williams Guide Receiver Sight). The first two attach on the left side of the receiver, the WGRS is fixed on top using the back two scope mount holes. I haven't used the newer types that all mount from the top.
I much prefer the Foolproof. It requires a screwdriver to adjust, thereby protecting you from accidental changes in setting. You can order finger knobs if those are preferred but I have never had them. They will clutter up your peripheral vision a bit more. It has click adjustments, is rugged and reliable. There are a variety of sized of aperture available and for hunting it works just fine with no aperture inserted at all...giving a ghost ring effect.
The 5D doesn't have click adjustments and is a little aggravating to adjust. Once set it is fine until you change loads and have to sight in again. The WGRS is the most trim and clean but it really is a pain to sight in. If you settle on one load for all seasons, it works just as well as the others. But as soon as you loosen the set screws the aperture will slip a little and then you've lost you starting point.
Just my experience with a couple of Marlins, a couple of W94s and Williams receiver sights. Front beads, blades and Firesights are another whole issue. Good luck with your search.