On paper Obliterators are probably the best CSM dakka, but for some reason I always roll 1s for their saves. It's maddening.
I'm honestly not sure I would use either of those setups. They're alright, but like they've pointed out, against high armor the autocannons won't do much, and the heavy bolters will be wasted.
I find that predators work best when specialized to compensate for something I'm lacking. Usually that will be anti-tank, since I'm a noise marine fanatic, so an all lascannon pred can be nice there. If you want horde killing, autocannon/heavy bolter is awesome. The problem with lascannon/heavy bolter is that you have two guns good at different things, so no matter what you shoot at one of your shots will be wasted in some way unless you're shooting at an MC or high toughness infantry.
If you want a take-all-comers support choice and want to avoid Obliterators, I would strongly consider Havocs with missile launchers. That way you can just decide what mode you shoot it in based on what you shoot. I'm also a big fan of Defilers with 2 CC weapons. Their guns might not hit as hard as vindicators, but they're still high enough to be awesome, much longer range and if you don't wanna pie plate something you can fleet it up and smack things around. That and they're something loyalist marines can't take, and anything that shows that we're not just like the 9198751978761861 Space Marine armies out there is a win in my book just for being on the table.
I'm personally not a big fan of autocannons. At S7 it's just enough to be too low to reliably be an anti-tank weapon, while only having two shots means you have much better choices against most infantry. The only thing is strikes me as being really good at doing is taking out monstrous creatures, which means it's really only good against Tyranids and Daemon armies that take Princes over Soul Grinders. At least that's my view on them, I've been wrong before.
So, if it's between those two with no other options, I'd go with the predators. At least one of their guns will be useful whatever you're shooting it at.