I am not a big fan of variable ND filters for a couple of reasons. First and, as you mentioned, there can be color shifts and other defects with the cheaper filters, and second, these are screw mount filters so they will only fit one-sized lens. I also find it much easier to change my exposure in camera rather than fool around with these variable type ND's. That is why I prefer the rectangular ND filters. They can also be stacked, if necessary.
If you are wanting an ND for long exposures, you will probably want to get a 3 or 4 stop filter, and you will want to get the best one that you can. The cheaper ND's can have pretty bad color shifts (ie, they are not really neutral), and fixing it in post can be a real pain. I would look at Lee and Singh Ray. I would stay away from Cokin. Also, beware of the 8 and 9-stop ND's from any manufacturer unless you can try the filter before buying. Making an 8-stop filter that is truly neutral is very difficult.
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from Down in the Valley
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