Hi Brant,
I'm new to the filter game, but it's been taking me a really long time to acquire the entire kit since the Lee products appear to be scarce in the United States. I'm liking Lee because after doing all of my research by scouring many internet reviews and forums. My findings guided me to Lee and Sing-Ray as being the best. I've never tried the Cokin filters, but there are many users out there claiming that they have the worst color casts of this bunch.
So, I bought the Lee Soft and Hard filter kit which was also a lot cheaper than buying a full set of hard and soft Singh-ray filters. I also bought one Singh-ray 3-stop reverse ND Grad.
When I hold both filters up to the sky, the lee filter appears totally neutral to me, in that it just seems to block out the light and increase exposure, the sky looks bluer and the clouds look whiter, when I hold the sing-ray up to the sky, the sky appears bluer however the clouds appeared a little bluer as well. The Singh-ray pretties up the picture a bit, which can be a very nice feature, but if you're truly after "neutral" Density than in my opinion the Lee filters are more neutral, especially when looking at the whiter clouds or whiter breaking water of the ocean surf. The differences are very subtle. I would definitely buy more Singh-Ray filters, especially the revere ones which Lee doesn't offer. You can always add saturation later.
I'm an audiophile as he may recall. There are some speakers and equipment that warm up the sound and make it sound darker or brighter, however as a true Audiophile, I'm always looking for the Absolute Sound and truest, most neutral reproduction of the music to start with, just as it was recorded in the studio or on the stage.
Rich