You should never use just a cable lock, but using a cable lock in addition to a u-lock is much more effective than the u-lock alone.
I find it hard to believe you're going places where a mini u-lock can't be used to lock up. That's weird.
how the hell are you supposed to lock a rear/front wheel and the frame and put it around something w/ a lock that barely fits the rear wheel and seat tube? assuming not using a cable. maybe im off on my sizing judgment. right now my u-lock is 4"x10" and its definitely a little big. I figure 4"x8" will give me a few more options for places to lock up to than a 3.25"x6" idk....
edit: plus i have a mtb that would be much easier locking up w/ the bigger one....guess i answered my own question there
thanks for the quick replies!