In our experiments, it is very hard to cut the cable with a dull cutter, but we can cut a cheap padlock without any problems. A sharp cutter will go through everything. It might be better to buy a cable with a lock built in. This will eliminate the weak link. For U locks, don't buy a lock bigger than you need.
Some thieves like to use that extra space and put their tools there.
If you don't leave extra room, then they can't use those tools. They might get stolen, but at least you won't cry as hard. If you have a lot of allen key bolts, you can try putting some epoxy into the bolts. It will take about 20 minutes to work it out, and that is too long for thieves. Don't try this the first time on the bike, try this on a bolt that is off the bike.