
Haltis tend to ride up into the eyes more on very pully dogs. I've used both - I used a GL on a spaniel boy (just about the strongest puller I've known aside from my youngun now) and it was very good; at the moment I use haltis (because I got them free! :-P), they are fine on two of the three I use them on because they don't pull much but on the littlun - who pulls like a steam train come what may - she spends most of a lead-walk with only semi-vision in her left eye (she walks on my right) because it rides up so much. Would go for a GL every time if I had a tenner spare!
Incidentally, I was advised to try a harness and headcollar combination for my big dog - she used to be an absolute 'mare on walks, terrible puller, no self control whatsoever, so on and so forth. The idea is that with a harness and headcollar, you have two points of control (so it balances the dog), and regaining control is much easier because you can alternate between gentle pressure on the headcollar and the harness. Usually best with a double-ended lead, although I did it with a 6ft lead with a carabiner clip at the handle end attached to the harness.
It worked wonders - she now has the headcollar on for security only, and because she's a total b*tch for snaffling food off the pavement. When she spots it she lunges - and she is a 44kg dobermann so has a bit of pull, especially when she catches me off guard! :-P
Edit to add: I have also used the canny collar but find its effectiveness depends on the dog. A friend uses it to great effect on her GSD, and it is fabulous for that dog, but it didn't work for Soli - she figured out in literally 2 minutes that if she moved herself just so, quick enough, the nose loop came loose enough for her to slip it off and carry on pulling. But I have seen it work very well on a few dogs!