Lee filter holder by far. I had (at same time) a Lee filter holder and a Cokin Z holder. Also had a hitech 4x5 soft grad and a Lee 4x6 soft grad. The Cokin Z could hold either grads, but I found it a pain to hold the grads in place with the screw tightener (which also damages the sides of the grads by the way). Putting the grads into the Cokin Z were a pain too. The Cokin Z adaptor ring and holder are made of cheap plastic that I could snap in half with relative ease. The Cokin adaptor ring sort of "attaches" to the lense.
The lee holder was much much better quality. Strong plastic frame with a metal lever pin that pulls out so you can easily attach the adaptor ring to it. The adaptor ring is metal, unlike that cheap plastic Cokin Z one, and scews onto the lense. All you do to hold the grads in the lee filter is just shove them into the slot. The tension holds the grads firm. You can adust the grads by pushin/pulling the ends. Not screws to tighten or loosen like the Cokin Z.
If you can only spent max 140 bucks on a grad system, well, the Cokin Z pro (don't bother with the Cokin P system. It's worthless for any wide or ultra wide angle lenses unless you decide to become michaelangelo with a knife) with the hitech filter will work. It's better than no grad system by far. If it was the only system I tried, I guess I wouldn't complain because I wouldn't know how the other system performed. Having the Lee system to directly compare the Cokin Z system against, really showed how much easier the Lee system was to use.
If you can spare the cash, go with the Lee! You get quality, functionality, extendability (you can add more stuff to lee system) and ease of use.
Also, I found the Lee 4x6 soft 2 stop soft grad far superior to the 4x5 Hitech 2 stop soft grad. The Lee gave, imho (you might find other people who tell you otherwise, but I actually set up a test (on tripod) and took shots with each filter), allowed much better image quality and the pictures were neutral. The hitech grads gave the images a greysih cast. Also, using 4x6 filters makes it MUCH MUCH easier to correctly position the grad and handling of the filters is easier. The cost is, well the monitary price (4x6 are more expensive) and they are harder to store.
Hope that helps