I started off with the Hitech filters and you know what I still recommend them, they did the job in the Lee Holder, but there was a but, they did suffer from a slight amount of colour casting when stacking filters together, and the ND 0.9 did have a colour cast effect when used in conjuction with their grads.
I also have the hitech 10 stop pro and couldnt be happier, works well, but I have replaced all my main ND grads with Lee and the hitech 0.9 ND with a Lee ND 0.9 Proglass, and yes the Lees do not have colour casting when stacked.... you get what you pay for I think.
Looking back, I ought to have just gone for Lee filters, but thats part of the journey in someways
andy