Axis one: input rules
Some services accept only a self-image; others accept any consented adult subject; others accept text only. The rule determines whether the product can do the job you have in mind, so it belongs first, before quality.
Axis two: motion magnitude that survives
Ask which template families a service handles cleanly. A generator that is excellent on idle portraits but breaks on full-body turns is not comparable to one that handles both — unless your project is portraits.
Axis three: cost per kept clip
Ignore the credit list price. Compare session spend divided by kept clips. This single number normalises for retry behaviour, which varies more between services than price does.
Axis four: deletion and retention
Whether you can delete an upload, whether deletion is immediate, and how long outputs are retained are product features. They are also the features most often omitted from comparison tables, which is precisely why they matter.
Axis five: change velocity
Adult AI services update model backends frequently. A comparison is a snapshot with a date on it. Treat any comparison older than a quarter as a starting hypothesis, not a finding.
A comparison is only as good as its weakest axis. Most are missing two of these five entirely.