Yes, Regirock does get Drain Punch. It was quite a popular option in previous gens.The things that Regirock does better than Conk are good enough to warrant it a spot on teams that either could use a deceptively spongy pokemon, or a pokemon that can beat out some of the above pokemon that Regirock can handle well.
If you are simply looking of an Av user, and synergy isn't in favour for the other, use Conk. Done.
Also, Regirock does get Drain Punch right? I just want to make sure :p
I don't think that Conkeldurr and Regirock should be compared. Aside from being AV candidates, they're completely different. You're not going to run Regirock because you want an Assault Vest user (or any other Poke, for that matter), you're running it because you want a Rock-type.
Hey, stop using that EV spread!252 SpA Mega Charizard Y Fire Blast vs. 252 HP / 4 SpD Assault Vest Conkeldurr in Sun: 297-351 (71.7 - 84.7%) -- guaranteed 2HKO
252 SpA Mega Charizard Y Fire Blast vs. 252 HP / 4 SpD Assault Vest Conkeldurr in Sun: 71.7 - 84.7%
252 SpA Mega Charizard Y Fire Blast vs. 0 HP / 252 SpD Assault Vest Conkeldurr in Sun: 61.8 - 72.9%
Although:
+1 252 Atk Tough Claws Mega Charizard X Dragon Claw vs. 0 HP / 0 Def Conkeldurr: 75.4 - 89.1%
+1 252 Atk Tough Claws Mega Charizard X Dragon Claw vs. 252 HP / 0 Def Conkeldurr: 64 - 75.6%
AV Conk isn't really cut out to be a physical tank, IMO. It can take strong neutral hits and retaliate with Drain Punch, but it's not worth sacrificing the special bulk, IMO.