I only have hands on experience with Berk but just judging from the pictures it is definitely a difference in manufacturing process that is making their product more expensive. Essentially the material quality and manufacturing quality makes the Berk product more expensive. The Garageline part has the appearance of a part that is mass produced somewhere for very little money (Chinese made knock offs). They are able to offer a completely polished tig welded stainless steel test pipe set for $170 which means very little was invested in the actual design of the product. Structurally the Berk is more robust thanks to the additional bracing welded on the test pipes. I would also be willing to go out on a limb and say that the Berk pipes use thicker walled material.
If it was my Z I would be going Berk Technologies for sure. Again my experience is biased as I have never held a garageline product, but Berk has never disappointed.
Thanks,
Alex Goodwin
AlexG@motionlabtuning.com