Bicycle cable lock for me, mostly for emergencies. Easy to to store, doesn't take up too much space and can loop it through my jacket if really needed also. I know it's far from fool proof, but I feel better at least trying.
There's a helmet lock built into the CTX700. It's under the seat, so that's the lock. The bike would come with a plastic wrapped cable with 2 loops. There's a place to connect it on the left side.