I watched a video from youtube where lockpicking lawyer recommended a Kryptonite chain lock due to being harder to break into with an angle grinder because it's not stable versus an U-lock.
Does this make chain lock superior to U lock since an angle grinder is the only way to break either locks but with the chain lock it's harder?
U-locks are vulnerable to angle grinders, which are noisy and slower, but also to nail guns, which are extremely fast.
Here is a video showing someone destroying a U-lock with a nail-gun that uses .22 cal blank cartridges to knock a piston into the lock mechanism:
You need to be careful with a U-lock to have a really good one and arrange it in a special way that makes breaking with a nail gun harder.
Chain locks can be cut by sufficiently strong cutters, but you may hope that the thief who just strolls around has only some weaker ones and will choose a different bike that only has a wire lock. Also, the cutters are bigger and harder to hide when working with them.
Depends totally on what tools your local bike thief is carrying at that moment.
If the miscreant has a jack/ram then the U lock can be forced apart quietly in a minute.
If the blaggard has an angle grinder, both a chain and U lock can be cut, but it's noisy and obvious. The chain will need to be held in one hand while cutting with the other, and the black nylon outer sock may create a snagging hazard.
A common tool for the discerning light-finger is the large folding bolt-cutters, where a 36" or 48" unit can fold up to about half that and be concealed under a jacket. These are also mostly silent and can cut most chain on the lower end of the spectrum They can even cut cheaper U locks.
From https://www.rakdistribution.net/36-inch-folding-bolt-cutters
One U lock can't secure the front wheel to the frame at the same time as the rear wheel, so you need something else. A chain helps extend the range of your locks. Two U locks might work well also, but they may not reach around fixed locking points like a rack or a wall shackle. A fence post or tree would be right-out.
Every bike can be stolen eventually, regardless of how many locks you put on. Your best options are to make it less-attractive, or take the bike inside with you.
U-locks tend to have much thicker shackles while being lighter and more compact.
In my opinion the only advantage of chains is that they tend to be longer and flexible which makes it possible to wrap them around bigger objects or more components of your bike. For example you can put most chains through the front wheel, through the frame triangle and around a tree.
If you tension a chain you can cut it with an angle grinder all the same while a short U-lock can be hard to get to without risking damage to the bike. So it very much depends on the circumstances and how exactly the lock is used.
Personally I quite trust and like my Abus Futura 64. It has an 11mm shackle while “only” weighing 726g. The compact size is both an advantage and a disadvantage. Chains with that kind of thickness tend to weigh several kilograms.
A chain-lock will provide better security than an equivalent U-Lock.
A good quality chain-lock (e.g. Sold Secure Gold Level or higher) would only be vulnerable to an axel-grinder (which unfortunately thanks to battery-powered grinders is a possibility with professional thieves), which isn't quiet and should take at least a minute.
It's possible to lever open a U-lock, which is usually quicker and quieter.
Drawbacks of chain-locks are weight and transportation.
As an aside, insurance policies often define a minimum rating of lock used.
Rule of thumb is the lock should cost 10% of the bike price.
The best protection, from a cynical perspective, is parking next to a nicer looking/more expensive bike which has a cheap lock...
