I would recommend the Kensington lock since it feels durable and has a rubber mount where it meets the side of the powerbook, meaning it doesn't scratch it and it has no slop in there. I had a friend with a combination lock and the numbers locked up on it so the company told me I had to cut it off with a hacksaw. I wasn't too happy about that.
Yeah, that was big news in the bicycle world last year sometime... you can probably find some unsettling videos of people picking expensive locks using only a ballpoint pen tubeall in a matter of seconds!