Lens: Screw On vs. Bayonett
 

what are the advantages/dis-advantages of both?

Screw ons can be misthreaded. They might not screw on "completely" which leaves the rear element an undetermined distance from the focal plane.

Bayonets are quicker to change, and consistently sit at the right focal distance.

Yeah, but only being able to use the adapter on one specific model of camera is a serious drawback. You'd have to be a total lunkhead to misthread a screw on, although I'm sure it happens.

Screw ins can easily be cross threaded, especially in the heat of battle.

If it's lenses, bayonette is the way to go. Imagine screw threading an eighteen inch long, 138mm diameter, twenty-four pound lens onto a camera and the Director is yelling at the DP, it's windy, the DP is breathing down your neck to move it because the last little sliver of daylight is about to slide away and it's a one minute take, the Camera Operator is miserable and glaring at you, the matte box keeps getting in the way, it's 28 degrees and raining, you can't feel your hands, it's icy...

You have a definite point, but I think on these forums people would mainly be dealing with much smaller wide angle and anamorphic adapters with 58mm and 72mm threads. Personally, I'd rather not invest in an adapter that could only be used with a single camera model. I also tend to leave them on all the time, so swapping them out has never been much of an issue. Other people may have different shooting styles I guess.

True, but I think things like the century .7 W/A adapter are pretty specific to each lens, i.e. depending on the manufacturer and lens, the mount may be different. Other than standard screw-in threading, the bayonettes vary from manufacturer, to manufacturer. In the case of the Century converter it is bayonette for the Canons. There are no threads on the front of the converter, so you are left with the matte box option, or some type of clamp-on solution

If we're talking filters, for a very serious amateur it may be wise to get 4x4 glass. Resin is cheaper, but I think it may be more susceptible to scratches. If you are not that serious, you may want to go with one set of screw-in filters that will fit your still camera and your video camera. That may be 77mm, or there abouts.

Regardless, you should standardize to one size if possible.
The caveat to that is that if the manufacturer decides that they are going to have lenses with a larger front diameter you are screwed. Hee, hee, I had to work that into the thread, this thing needs a little twist.

As an example, when I was shooting stills, I standardized all my filters to 72mm. Most of the front diameters of the Nikon lenses were 52mm. So, I had a bunch of step up rings 52mm-72mm, and 62-72mm, so that if I was shooting, I could have several lenses ready to go with the step up ring on and all I would have to do is switch filters to the new lens, when I changed lenses.

That was great for a few years until Nikon decided that the new everything zooms would be 77mm front diameters. The 3 inch Wratten gels I had were kind of kaput too. Man, that still hurts. Big, full sets of gels too, CC, LB-Ouch.

Long story short, for a basic filter, sure a screw in is fine. It's more common and you can get step up rings for cheap. Bayonette, is less common and I think most of the prosumer lenses have screw in threads anyway. You could get step up rings and standardize to Series 9. But it depends on how serious you want to get.

Take a look at the equipment you have, look at what else is available and how you plan to expand. Take into account that things may change. How long have you had your rig and how much longer will you use it? Upgrading soon?

Just take your time threading those filters. A quick tip, rotate the filter backwards and you can feel/hear it drop into place and then screw it in the right way. That will help prevent cross threading.