In my opinion, if you're spending money on a lock, just get a really good one... there is a very large gap on quality(type of steel, disc locks and etc) and deterrence(image of being a tough lock) between the expensive locks and the no-name. Why spend half the cost on something that only gives a quarter the protection..
They all are easily defeated, its like locking a convertible car with a lap top on its seat.
But I thought it was clear that even the biggest and best lock don't stop a thief, so why carry around a heavy lock?
I don't think a lock is a good place to skimp. I mean it's not like you're really saving much anyway. Just pay the few extra bucks for a nicer lock to lock up your clunker. It would suck to have to walk home.
Look man this isn't complicated. Either ride a clunker, which is what I would do, and use one really good U lock like the Krpto NY, and then add maybe the MasterLock Handcuffs or even a really thick cable lock that you can run through both wheels, frame and seat as an option. But at least get the best U lock because even if you ride a clunker, then later when your on your good bike but decide to stop somewhere to get a bite you have at least one really good lock you can use for that purpose without hauling two locks on a training ride.