You have to consider the risks and their probability. If you want to cover every possible risk all the time, you'd need all sorts of heavy hardware that you'd have to carry while riding, which will spoil your ride.
If you go out for country rides and have a break at a village pub, the chance of a thief passing by is not very high and the chance of a professional thief taking an interest is nearly zero, so for those circumstances, a medium cable lock would be more than enough. Locking two bikes together makes them more or less impossible to move, but it's always better if you can run the cable around something immovable.
Parking your bike in London in the same place every day while you're at work is going to get it nicked sooner or later regardless of which locks you use, but the more locks you use, the longer it will take thieves to get through, so a lot of locks would probably make them prefer to look elsewhere. Here, the number of locks is probably more important than the absolute quality, through the harder they are to cut, the longer it will take and the more they're likely to be put off.
Parking outside the gym for an hour once a week is also high risk. The thieves sit in cars waiting for someone to park their bike. Once the owner has gone in, they jump out with bolt-croppers. In that case your lock only needs to resist bolt-croppers.
Basically, you need to consider the circumstances that you're likely to leave the bikes. In many cases, you can keep them more or less in sight, which will be better security than any lock.
I use my bikes for rides out visiting people, shopping and going to pubs. I see those as medium to low risk situations, so I've never used more than a cable roller lock. It will defeat all casual and opportunist thieves. It's not too heavy, not very expensive and very convenient to carry and use. Mine is no longer made. the nearest equivalent is the Squire Mako Plus 25/1200. You only need one for two bikes because it's long enough to go through two frames and round a lamppost or whatever.
