There are a number of differences. Its like comparing some strange unknown brand bicycle from a department store to a high-end Cannondale. Yes, both have two wheels, but there are a number of differences:

Lock cylinder. Likely the Bell will have a 4-pin tumbler lock, or something that is cheap to make. The key may even have a plastic cover on the bow. Looks decent, but any good thief who knows how to pick locks will have it open in seconds. Any decent U-lock will most likely have an Abus/Abloy clone cylinder [1]. Yes, it *is* pickable, but its time consuming and there are no known ways to easily pick it with some pick gun.

Shackle hardness. Yes, all shackles will fall to power tools, but its a time difference. Time is against thieves side. If a U-lock is cuttable with a set of bolt cutters, it will be cut and the bike taken compared to a U-lock that will require a grinder or a cutting torch. There are different types of hardened steel, some grades softer and easier to cut than others.

How the shackle is held in the lock. Cheap U-locks won't use anything on one side, medium grade will use a bent food (Kryptonite's staple for decades), high-end will have the locking mechanism lock both sides, forcing a thief to make two cuts to get the lock off, or have to cut/crush at the locking mechanism, where the lock is usually the thickest and most well-protected.

Drill resistance. Low-end U-locks have none, medium and higher have discs or ball bearings which will spin.

Bumping/Ace Lock resistance. Higher end U-locks are resistant to the "backdoors" found in a number of locks these days, be it pick guns, bump keys, the much talked about Bic Pen, or shimming.

With everything pretty much, you get what you pay for. A $10 lock will keep a kid brother off a bike. A $100 lock will keep all but a dedicated professional thief whose living is bike theft off your bike. Assuming the locks are used properly.


[1]: For the details on Abus/Abloy cylinders, go to www.lockpicking101.com, and search on abloy. There are a good amount of threads. Abloy locks have been around since 1917, and to pick them, its less picking, than figuring out the detainer disk combination.