More important than the lock is how you use it.
Rear wheel, through the rear triangle. If you wish to secure the front wheel, either another U-Lock or a very thick braided cable lock, with a separate captive shackle lock (abus makes one that is possibly the heavily amrored captive shackle I have ever seen). If you run the braided cable through the u-lock arms, it just means they only have to break one lock to ride off with it, but with a separate lock on that cable, it makes the number two locks, and two types of tools to ride off with your stuff.
I've had good luck here just using the cable through the u-lock, but I tend to lock my bike in places where theft is pretty low. If I go into a shadier area, the 7lb chain and the captive shackle lock comes out...the u-lock becomes my front wheel lock.