When I do errands I use a pair of locks. One is a cheap key-lock cable. It is thick, but anyone with a steak knife could saw through it in several minutes (it would take determination, at least.) The other is a hardened chain with a gym lock---no one is cutting the chain without at least two feet of bolt cutter or a grinder, but someone who is determined can probably guess the combination in five or six minutes. i figure no casual thief is going to want to do both mental and physical labor, nor have the tools, nor be shopping to stealable bikes anywhere I lock up.

I never lock a bike anyone could sell for real money---I figure if I beat the crack- and meth-heads and the casual joy-riders I am doing well enough. I only park in places where someone messing with the bike would be noticed---so at least maybe someone will post a video of my bike being stolen on YouTube---and only in parts of town where i feel anyone really sketchy would stand out---and where pickings would be so slim not many people would bother looking for stealable bikes there.

You could spend hundreds of dollars getting the best u-locks (Fahgedaboutit seems to rate high) but a cheap u-lock will stop anyone who can't beat the best u-lock---if the thief has a grinder or a small hydraulic jack your bike is going away. You can spend hundreds on the best key locks with square-link, ultra-hardened chain---same deal. And hardened chain will beat any thief except the thief who can beat any chain.

In both cases, if you ride a light bike, the security system will add ten pounds, so why not buy a cheesy $100 Schwinn--or three or four? That way you walk home one night (or Uber,) and the next day you are back on another cheap bike.

I suggest going to the hardware store and getting two four-foot sections of hardened steal chain and a couple of hefty-looking locks. (I slide my chin inside of old inner tubes to prevent scratching.) The most important thing, IMO, is that your bike Looks secure to would be thieves ... and even more important, that you Feel secure when you leave it.

Some folks Need to spend $400 on 15 pounds locks, chains, and u-locks just to Feel safe. Some folks use a $5 cable with a built-in combination lock ( I used one for a couple years.) "Security" is really a feeling, not a state or condition. Your bike is never "secure" (there are posters were who have had bikes stolen from their apartments.) Decide how much you need to spend to have peace of mind, the google away.