I carry a $30 U-lock and cheaper cable lock. While chain might be more secure, it's too bulky and heavy. The last time my bike was stolen they cut the lock itself not the cable. I had a U-lock but it wasn't on my bike that day. I was only in the convenience store for 2 min.

In 2010 I was taking night classes at a local community college. I had a 15 yr old Raleigh Mtn Bike worth less than $200. At the time I was using just a cheap cable and lock. I had parked all over with that and no problems. It went thru most of the semester just fine. Then one night after class it was gone. In that case the cable was cut.

As to the likeliness of your bike being stolen. It's always a low probability each time you lock it. But there is no such thing as certainty. Nowadays I try to park my bike where I can see it. If I can't I think about it. Sometimes I go to another business where I feel my bike is safer.