The gorkanaut is a close combat beast. Nothing compares to it. It competes directly against the imperial knights for raw damage output. (better CC, worse shooting).
The Meka dread is good at tying up enemy forces (mega charga), and monster/vehicle killing. It suffers due to it's T7 (T8 is really really good) which reduces its survivability, and it is not as good as killing a mass of troops. Two gorks can wipe an entire board quadrant by themselves easily. 36 CC attacks is no joke. The rattler cannon is good on the meka, but it's very variable due to the 2d6 shots. (sometimes it does nothing).
To give you an idea of the raw combat potential of two gorkanauts. A full strength unit of 30 cultists, rhino, 10+ khorne beserkers and buffing characters...all wiped in a single turn by two gorks. Like a gaping hole in the battlefield that was.
I really think when the mega dread comes out, we will have our favorite dread. Double heavy skorcha's and a supa skorcha will make it the most terrifying dread on the battlefield. 4d6 auto hits a turn and when you charge it? Yes please.