Get a U-lock, they run $15-30. They are virtually unbreakable- they'd (thieves) need a grinder and work on it for at least a few minutes (that horrible high pitched screech and light show would wake up the average guy at home or at least alert a pedestrain that "something" is going down (if you lived in a smaller town this would be especially helpful). Basically, no one's gonna bust out a grinder to make a lotta noise and fireworks to steal a $200-500 bike. We have premiere carbon fiber Specialized, Bianchi, etc..$2,000+ bikes here in Seattle and people lock them routinely with U-locks on the street without incident.
Now a cable or chain with a padlock? I could get that sh*t off in 5-45 sec. gaurenteed everytime regardless of size or lock. With a pair of pliers or a bolt cutter- and a nice motorized bicycle with a filmsy cable or master-lock and chain is going to be attractive to a theft.
Make sure you lubricate the U-lock key hole routinely, those keys aren't strong (never force the key if it doesn't turn, just try again).