For those considering buying filters, consider this.
When you watch 3D, you will lose between 70/80% light output (approx). With a ND4 filter, you will lose 75% light output. So, if you want a filter and you want your 2D to be about the same brightness as 3D then buy an ND4 filter as you will already know what this looks like. If you want your 2D brighter, then go for the ND2 filter.
Obviously you need to remove the filter when watching 3D or else you'll be down to about 5% light output