Dude, are you serious? Pursuit means you're out of the game and Mach Punch means "oh no, Conkeldurr can revenge me if I'm low enough" (which is 51.5% - 60.7% btw). There is a huge difference between being able to switch out of a priority move safely, and getting boned on your first kill if your opponent happens to have the right Pokemon.
Also, Hydregion's movepool is just way, way better. Seriously, U-turn, Fire Blast, and Focus Blast are all awesome and have kick-ass coverage. He can run STAB Dark to check Rank and whatnot if he wants, but the number of things that can safely switch into Specs Hydregion is about the same number as the ones who can switch into Latios, sans Blissey. Latios can be faster and specially bulkier if he wants to, but for me, it's either Specs or no go; that Pursuit weak is kind of a big deal.