Definitely a combination lock for me. I use all the same combination on all my combination locks so I only have to remember one combination (for most you can reset the combination to whatever you want). A cable lock just has a reputation for being so easy to cut, I see no point in the additional hassle of a key.

I only use a cable lock in 2 general situations -

1. Just to make sure someone doesn't walk off with it in 2 seconds. For example, I'm at a restaurant where I can see the bike (I don't live in NYC or something crazy like that). Or where I work, the bike rack is monitored by security guards and video and it's a good neighborhood - sometimes people leave their bikes unlocked on the stand, and other people use a cable lock with very expensive bikes. Or on group bike rides where other people often leave there bikes just sitting on the grass without a lock at all.

2. In conjunction with a ulock - say, to lock up the front wheel, but the frame is locked with a ulock. In this case, keeping track of 2 different keys for 2 different locks is a pita, I prefer the combination lock.

If you have a combination lock that someone was able to jerk open, it is entirely possible that the same level of key lock might be broken from the same action as well.