The reality is all carriable bike locks of any reasonable weight are going to only keep the honest people honest - search the internet for "Lockpicking Lawyer" and you'll see most lock systems being easily broken through, often with smaller tools than gigantic bolt cutters.

I've always found U Locks harder to carry than cable locks, so I've mostly used cable locks. I try never to leave a bike outdoors overnight but every now and then have to do so on my car - I carry a big heavy chain and padlock for that but never carry that while riding!