I would say that either is fine. I think tags like these should be considered synonymous. The only situations in which one valid spelling is prioritised over another should be when
- Existing convention dictates it (such as "HTTP Referer," which I don't think is actually valid in any culture but still used by convention).
- The term is a proper noun; product names etc.
- The term has a product-specific meaning, e.g. 'localization'.
As "content modelling" is a general term, I don't think any restriction should applyit matters too much. If they are synonymsanything, one is likely to emerge naturally asI'd go with the de facto "correct" tag anyway, once autocomplete has its sayAmerican-English version for the sake of consistency.