Warren. While experience teaches us that we never know what the hell we're gonna get once they're actually in office, the thing about Hillary is that we already know she's going to just continue a lot of the current crap that's going on, and she's not an actual progressive. She's a moderate who's not terribly concerned with change or even efficacy, like nearly all Dems these days.
Warren is a progressive, and has a very good track record of such. She has changed very little since coming to national prominence. All of that is a good sign.
That could still change as soon as she's occupying our highest office -- it has with many presidents before. But worst case scenario, we just get the same thing that we would have gotten with Hillary. I'd rather take the risk than settle for the known mediocrity.