Both OmegaWiki and Semantic MediaWiki are providing semantic support. There are people that have expressed that OmegaWiki should not include particular types of data because Semantic MediaWiki does a better job.
Both OW and SMW are extensions to MediaWiki, so at first face it seems like a reasonable suggestion. The two extensions however do completely different things.
Semantic MediaWiki will shine when it becomes part of a project like the English Wikipedia; when key data that is in the article is marked, it will provide a great improvement in making these facts available. SMW even provides a really rich environment to query the information. It is absolutely great and it is absolutely mono-lingual.
OmegaWiki is at this moment very much a stand alone application. It does not derive data from anything, it is great at presenting the same data in many languages. This means that when we know that the information exists, we will show it in "your" language.
SMW can export and when OW can import, we have the best of both worlds; that is to say we have the best of both worlds when we can link the resources to each other. As OmegaWiki is not encyclopaedic and does not want to be, it is our stated intention to link to Wikipedia articles. As the SMW is tightly linked to the Wikipedia articles, this may be just the trick.
The suggestion that OmegaWiki would only have the semantic information that is in Wikipedia is incorrect. In Wikiprotein we already have a real rich set of annotations of proteins. This is the kind of information that is not encyclopaedic. It enables scientists to maintain information on "their" proteins. The language of the science of proteins is English, however many are known in other languages. It is rich that all this can integrate as it brings diverse information together.
Both OmegaWiki and Semantic MediaWiki have their own, and different strengths. Within Open Progress we use Semantic MediaWiki for our internal wiki. It works absolutely fabulous. I love both MediaWiki extensions!