I have also recently been researching the same subject. I'm a DSLR user, but there are many scenarios where shooting without an ND filter is just not possible. In my extensive exploration of ND filters, I've found Lee Filters. Both from a textbook technical perspective, and in reality, Lee seems to have the best filters available.

Some of the things I've learned about ND filters are that you need flexibility in how they are situated in front of your lens (particularly for ND Grad filters), and quality of filtration. A lot of other brands don't filter all ranges of light...they filter the "central" bulk of visible light, but allow either infrared or ultraviolet wavelengths to slip through. This can give an undesired color cast to your images...either warming them or cooling them.

Lee filters are solid filters that don't leave much of a color cast, and they have a very broad range of stops, anything from a few stops up through their "Big Stopper", a whopping 10-stop filter.

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In regards to which kind of filter you get, you will ultimately probably need both solid and graduated filters. One of the most useful purposes for an ND Grad filter is to balance the contrast in a scene, and depending on the scene, you may want a hard ND grad, or a soft ND grad. If you have a sharp horizon with a very bright sky, a hard ND grad will serve you well. If you have a softer or less defined horizon, such as a mountain landscape, a soft ND grad will probably be better. The most common ND grad filters I have seen in use, and the ones I've purchased for myself, are the 0.6 and 0.9 (or 2 and 3 stop).

If you wish to greatly slow down the rate at which light reaches your sensor, by that 10-20 seconds, you'll likely need a high-stop solid ND filter. Lee's Big Stopper is an excellent high-stop ND filter that will definitely get the job done for you.