same lock in a combo style can be bought at REI for like $11. That's what I use. I believe it's 10mm cable and 4' long. Burly enough where you would need bolt cutters to break it and small enough to still wrap up nicely in your pocket
Go for a combo lock. I've got like 13 pockets between my jacket and pants when I ride. The last thing I need is be looking for a key. Just a little WD40 every few trips to avoid hassles. I have one of these Wordlock 4-Wheel Cable Bike Lock : Target
. Downside, you don't get to pick the word.
And as has been said on many of these threads, leave it locked on the rack at the base area.