I have had accounts with four stock libraries over about six years, across the main pricing models. The differences that matter are not the ones the marketing emphasises.
The pricing models and who they suit
Subscription with unlimited downloads. Fixed monthly cost, take as much as you like. Suits high volume work where you are pulling many clips per project.
Credit or pack based. Buy a number of downloads, use them when needed, generally with an expiry. Suits occasional use where a subscription would sit idle.
Per-clip purchase. Highest per item, no commitment. Suits someone who needs one specific thing once.
Free libraries, funded by other means. Variable quality, variable licence terms, and genuinely useful for a lot of work.
The mistake I made early was a subscription during a period when I used it twice a month. The per-clip cost worked out higher than buying individually, which is the opposite of what a subscription is supposed to achieve.
What the licence actually covers
The part nobody reads and the part that causes every real problem.
Standard licences generally cover use in a video distributed to an ordinary audience. What they frequently restrict is broadcast, distribution above a certain audience size, use in a product for resale, and use in anything trademarked.
The audience threshold is the one to check. Some standard licences specify a maximum distribution, above which an extended licence is required. If something you made unexpectedly reaches a large audience, you may be outside your licence without having done anything.
Also commonly restricted: using footage in a way that suggests endorsement, using footage of people in sensitive contexts, and using footage as the primary content rather than as an element.
That last one caught me. A compilation made largely of stock is, under several licences, not permitted, because the licence covers use within a work rather than as the work.
Release status varies and matters
Clips featuring recognisable people should have a model release, and clips of recognisable private property should have a property release.
Libraries indicate this and the indication is not always prominent. Footage marked editorial use only has no release and cannot be used commercially, which includes a lot of contexts people would not think of as commercial.
The practical risk is low for most uses and it is not zero, and it is entirely avoidable by checking the field before downloading.
Where the catalogues genuinely differ
Beyond size, which is a poor measure.
Aerial and landscape material is abundant everywhere and largely interchangeable.
Anything involving people doing specific things is where libraries differ, and where most of them are weak. The generic office and lifestyle material is plentiful and looks staged, because it is.
Specialist material — industrial process, medical, scientific, historical — is patchy and this is where a library either has what you need or does not.
And regional coverage is heavily skewed. Footage of certain cities is endless and footage of most of the world is thin, which is a problem if your work is about somewhere unfashionable.
The search is the actual product
Having used four, the difference in how quickly I find something usable is larger than the difference in what is available.
Good search offers filtering by duration, resolution, frame rate, orientation, whether there are people, and whether it is released. Poor search offers keywords and hope.
Similar-clip search is the feature I use most. Finding one clip that is nearly right and asking for more like it is faster than describing what you want in words.
Collections matter too. A library that lets you save candidates and compare them is enormously better for a project where you are assembling a consistent look.
Free libraries, honestly
Better than they were and worth using with two cautions.
The licence terms vary considerably between them and some are less permissive than the presentation suggests. Read them once per library.
And the material is used by everybody, constantly. If you use a popular free clip, there is a real chance a viewer has seen it in three other things that week.
Where free libraries are strongest is texture and abstraction — overlays, light leaks, natural motion, atmospheric material that supports rather than carries.
What I would do differently
Match the model to actual usage, measured rather than estimated. I ran the numbers after two years and had been paying for a subscription that cost more than per-clip purchase would have.
Read the licence once properly and keep a note of the distribution limits.
Check release status before downloading rather than before delivering.
And shoot more myself. A surprising amount of what I bought was generic material I could have captured in an afternoon, and the version I shoot is specific to the project and nobody else has it.
Keeping track of what you used
A habit worth adopting early, which I adopted late.
Keep a record, per project, of every stock clip used: the library, the clip identifier, the licence type, and the date downloaded.
This matters when a client asks whether they can reuse the video somewhere else, which they will, and the answer depends on what licence each element carries.
It also matters if a library changes terms or removes a clip, which happens, and if you ever need to prove that you licensed something properly.
A text file in the project folder is enough. Reconstructing it two years later from download history is possible and considerably less pleasant.