it's a matter of how long it'll take and how much noise it'll make.
Chains are quite nice for thieves as they make two loud pops nicely dispersed as it gets cropped and that's it.
Disc locks are a pain as you make a racket mashing them off and you knacker the front disc. They'll deter casual thieves and combined with an alarm and a big chain attached to something solid will probably make many serious thieves stealing to order look for an easier target.
I'd not leave my bike anywhere not chained. If it's not chained down then the chain is on the front wheel and balanced so anyone trying to move it will move the handlebars slightly, setting off the alarm sooner rather than later.
Yeh they can lift it into a soundproof van but if you park it at home somewhere where it'll take a while to get it to the van (mine is round the side and takes a good minute to get it anywhere near the road - a long time) then the only people you have to worry about are opportunists when you're out and about.
Bear in mind that someone I know had to legitimately steal his own bike. In the middle of town. He was there with a hack saw, drill, etc, etc with people walking past as well as cops and no one even questioned him.