Both those rigid U-locks and flexible cable locks should be functional enough for keeping a bicycle tethered to your favorite sturdy post. If you prefer something that can fold up into a compact bundle, however, the Foldylock might very well become your favorite bike lock yet.
Instead of a single-length of steel like a U-Lock, the contraption uses six different metal links that are attached with pivoting rivets on each end, forming a contiguous six-sided lock that can change in shape for a convenient fit. The links form a length of around 35.5 inches total, allowing it to secure the bike around even very chunky frames.
When not in use, the Foldylock unfolds to less than 6 inches in length, with its own case that can be mounted on the fixing screws in place of a bottle holder. If you'd rather keep the bottle cage on board for your Scosche BoomBottle, they also include a pair of specialty-straps that you can use to keep the Foldylock case mounted on the bike frame. Construction is hardened steel for the links, with fiberglass-reinforced plastic on the surface to keep the metal from scratching your bike's expensive aluminum frame. It weighs 3.3 pounds.
All the parts are rust-free, with anti-drilling cylinders in hardened steel housing. The rivets are all designed using VSR's rivet technology, which keeps them protected from any would-be thieves who dare attempt to break them up by drilling. Its creators claim the locks can meet any of the highest locking standards currently used in the market.
The Foldylock is currently collecting orders on Kickstarter. Pledges to reserve a unit starts at $80.