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 apply. If they are synonyms, one is likely to emerge naturally as the de facto "correct" tag anyway, once autocomplete has its say.