I used Hitech, but now use Lee (though still use the Hitech holder).
The Hitech filters, which are much better than cheap Cokin ones, still had some colour cast that I found more often than not difficult correct for. Doubly so if it's a dark ND Grad I'm using, as the cast obviously differs in different parts of the frame. With the exception of the Big Stopper, I've observed no colour cast with the Lee filters, and the big stopper is a quite easy to correct blueish hue.
Yes, Lee are expensive, but you get what you pay for IMO. Most professional landscape photographers I know wouldn't use anything else. As it's pretty likely they're also much more skilled in Photoshop than I am, have more time to perfect pictures in photoshop, and are probably much more critical about splashing their often meagre profits on gear, the fact that they all have no qualms on paying a lot for Lee over other brands speaks volumes.
As ever, YMMV, but I noticed a marked improvement in colour when trying a Lee and a Hitech side by side. It's doubly noticeable if using two filters e.g. ND for slow shutter, ND grad for the sky.