This is all about the environment you predominately want to lock your bike up in.  For example; central city vs local dairy.  All day/night vs quick stop.  Lots of people vs isolated.  High crime rate vs low crime rate.  Also be aware the more unique or expensive looking your bike is the more likely it will be a target for crime.  Theives will look for the “easiest” steal so how are the bikes around yours locked up?  Start by matching a type of bike lock to your situation.

What security rating do you need?

Bike locks are not created equally some are just deterrents for an opportunistic thief.  Meaning if it’s locked to something someone can’t grab it and ride away (it is estimated that 85% of bike theft is opportunistic), other locks are made to make breaking them difficult even with all the right tools.   Locks have security ratings from Diamond (very high) to Bronze (low) or a star rating, 5 stars (high), 1 star (low).  Generally the thicker the cable/steel the harder it is to cut so the better the rating – starting with highly rated chain locks, then U-Locks, dropping down to cable locks.

Read more at Bike Lock Security Ratings 

Is your bike integral to your daily life, is it how you move around your town or city getting to work/school/shopping?  If this is true invest in a quality lock.  Your bike is valuable (this is about your disposable income and how “costly” it would be to replace not the monetary value of the bike).  If this is true invest in a quality lock.  Have you invested a lot of time and/or money into your bike?  If this is true invest in a quality lock.  Buying a quality bike lock may involve saving up for something slightly over your immediate budget.  It may also involve you making a decision to use a higher security rated bike lock than the environment calls for because you want to have the lock that will be most difficult to break.

Locking 1 bike vs a group of bikes

Is this lock for a group of bikes?   For example; to take on a family ride, use on the car bike rack or at the end of a group ride while you explore/socialise further?  If so, you need length.  You will also need length if you often find yourself in situations where the “fixed object” you choose to lock your bike to wasn’t designed for this or the bike racks seem to always be full… power poles, trees, fence posts… a U-lock simply won’t work.

Read more at The Best Way to Lock a Bike , Bike Locks for Family or Group Riding

Bike locks are opened in a couple of different ways, either by using keys that you must carry with you or by entering a sequence of numbers into a combination barrel.  Both systems can be broken into, it depends on the skill set of the thief.  Also, this may not be where they choose to attack your lock if they have cutting tools.  So this decision comes down to personnel choice &/or the personality of the person using the lock – how many keys do you think your teenage boy could lose?  Is it easier to carry keys or remember a code?