Hey Tom, both filters are a great addition to the camera bag, and both serve different purposes. Well the same, but are usually used in different circumstances.
If all your lenses have the same filter thread diameter, then go with the glass filter type, B&W or Tiffen are my two favorites and a thin mount is even better if they offer it. This will solve any lens vignetting if you decide to stack filters or are using them on a very wide angle lens.
I have four lenses, my main four that all have a 77mm thread diameter, so I chose to buy screw on filters as I had the Cokin types for awhile, but kept scratching them (I'm pretty rough with my equipment
A regular ND filter is great if you want to effect to be used on the whole image. Say a waterfall shot during the daytime.
Grizzly Falls in Kings Canyon National Park This was shot at high noon in full sun and I was able to use 0.7sec shutter speed, ISO 100 f/13 because I had a .9 ND filter and a Polarizer on the lens!
Now if you're main objective is to shoot sunsets, a split ND filter or a Graduated one would be best. I have one of these also and use it about the same as the regular .9 ND filter. Sometimes I use them both at the same time. I seem to find that waterfalls, sunsets and anything where you're including the sky or bright sun in the image you almost always need a GND filter to make the image look like we see it in real life.
Tahquamenon Falls, Michigan's Upper Peninsula - I'll give the specifics of this shot away and you'll see exactly what I mean.
Again, shot in late afternoon sunlight on the river section of the falls which meant I had to stack both the .9 ND filter to be able to get the 1/6th Shutter speed needed to get the effect I wanted, but the part of the river that was in sunlight kept getting blown out. So I stacked the GND filter on top of the .9 ND filter and spun it so the filtered side was on the part of the image that had the sunlight in the shot. The shadowed section of the falls is now even to the part of the river that is in full sun.
There were probably 5 other photographers beside me shooting this same scene, and I heard everyone of them grumble to themselves after they shot the image that the upper section was blown out due to the sun or the falls was too dark due to the shadow. I even offered one guy my filters because I noticed his lens had the same size diameter as I had, but he just looked at me funny and said "No thanks!" I guess he wasnt as worried about getting the shot as I was? I've run into so many photographers that are affraid to try others equipment when offered. (I almost always offere any of my equipment if I see a fellow Canon shooter while out shooting)
I chose to go with the .9 ND Filter, as I figured the darker the better, and I can always just use a longer shutter speed, or bring it back up a little on the settings side to compensate, but I found that I was stacking my Polarizer on so may times to try to help cut the light, that I might as well go with the darkest one they offered. I'm not the type that is up at sunrise every morning to get the shots where the light is best. So I just buy some equipment that gives me that same light, no matter what time of day I'm there...LOL
This is one time I'd go with a top name. I have a off-brand ND filter, and when ever I use it, you know I used it
I have the best luck with B&W, but man are they exensive! I think the .9 ND thinmount was around $150! But well worth it if you shoot a lot of waterfalls, sunsets of anything that you need that different affect