An article is generally written to be read relatively easily within a website, publication or other editorial environment.
A white paper is usually a more substantial standalone document designed to explore a specific issue, argument or solution in greater depth.
White papers often contain more formal research, evidence, data or technical explanation. They may also be designed as downloadable assets for a particular professional audience.
Articles can still be detailed and authoritative, so the distinction is not simply word count.
The better choice depends on what the content needs to achieve.
If the objective is to explain an idea accessibly and reach a broader audience, an article may be enough.
If the subject requires a structured argument, substantial evidence or a document that readers can circulate internally, a white paper may be more appropriate.