I have two sets of Hitech. Never had Lee filters so I can't comment on them, 'though their good reputation goes before them. I also have a Cokin...
I put the Cokin and Hitech graduated filters on my densitometer and the Hitechs were all neutral but the Cokin showed some slight colour cast. As these are the only one's I have tested, all I can say is that *my* Hitech grad filters show no colour bias.
Given that Hitech cost close to a quarter the price of Lee for a set of 3 grads (£35 Vs £130 at Teamwork) and use the much cheaper Cokin 'P' holder (Hitech also make holders etc.) I chose Hitech over Lee, especially as I don't need the extensive Lee system that uses their holders too.
Of course, you could look at Formatt (who also manufacture Hitech) glass filters, but at £350+ for a set...
Cheers, Bob.