The differences between ND filters are not necessarily between kinds but rather between values.
The value you should look for is how many f-stops (or in this case shutter speeds) do you want to change.
Let's say the light conditions you are taking the photo in now (9AM, with sunlight, in summer, your iso and aperture well defined) give you a shutter speed of 1/500 and you need a exposure of 1/2 second.
The f-stop difference will be
So you need a filter that reduces the light 8 f-stops.
According to this table http://en.wikipedia.org/wiki/Neutral_density_filter you can use a ND108 or N119 filter.
It is not recommended that you stalk too many filters, but probably 2 will be fine, so you can buy for example a 103 and a 106 and combine them to have a 109.
Polarizing filters are helpful and they have an Xx scale to see the f-stop change. A filter that is marked 2x will reduce the f-stop 2 steps.