I highly recommend a lightweight U-Lock. Something like the Kryptonite commuter or similar. A cable lock can be cut through in 5 seconds using a pair of wire cutters purchased for 2 dollars at the dollar store.

I personally use a Kryptonite U-Lock New York Standard for every type of stop I make. Period. Remember, bike security is nothing but making it harder for the thief to take your bike, allowing them to pass it on and move to the next bike.

Don't skimp out on it. Unless you are in a very low threat area, even still, it only takes one person to eye up your bike in the right way.