U-lock or Disc lock?

[Slaninar]

Back it up with a chain and a BIG bright coloured good quality padlock on the rear wheel if its being left for any length of time, it doesn't really have to be a super heavy duty chain because it someone is going to cut it with a 18volt battery powered disk cutter they will whether it is a 8mm link or a 15mm link, but it is a visual deterrent to the little foc pigs who want to go joy riding. Make sure the chain is of a length that it cannot be laid on the ground as it is difficult to cut a hanging chain with a hammer and chisel. Always use the steering lock.
There is no way you will stop someone nicking your pride and joy if they are determined but you can make life difficult and that will stop the opportunist thief. 

In my knowledge and experience, there is a difference between an 8 and a 15 mm thick link. There is also difference between well hardened steel and cheaper, softer one.

The main breaking point, so to say, is about 12 mm of well hardened steel. From that thickness, it is very hard to cut it with tools that fit under one's jacket to stay invisible. Also, that is the thickness that requires more than a few seconds with a battery run angle grinder - so there will be more noise and light from all the grinding sparks (English term?  :/  ).

No useable system is 100% hack proof, but the goal is to make it more work and more risk for the thieves than the other bikes.

Of course, a garage is much better - out of sight, at least in my country.

Another good thing is being nice to the elderly. Those people NEVER sleep   and will always let you konw if they saw someone suspicios, or call you (or the police) if they hear some break&enter noise.