To me, the one-off nature of the authorization resolution is a positive feature. Basically, it's a stand-alone political fight, unencumbered by any extraneous details involving troops, Pentagon lobbying for funding, or anything else. Defunding is a messy thing, and I think the Democrats should avoid messy, if possible. Keep it simple and easy to understand. You may not have the numbers to pull it off in the Senate, and if so almost surely won't have the numbers to override the veto, but there it is. That's reality. But this has the best chance of success, imo, and by far the least downside of any approach.
Meanwhile, while the fight over actually continuing Iraq is confined to that one vehicle, you can use the funding mechanisms to go after other things, like the unbelievable corruption and profiteering that has, among other things, gone to fund the political establishment of the right-wing.
Just noticed an inconsistency in my last two posts in the chance of success. What I meant in the first is that you have the same target for success of both approaches (a veto-proof majority), but, as I mention in the second, you have a better chance of reaching that with the de-authorization approach. It's still not much of a chance, admittedly, but it's at least better than going after funding.
