Python or Ruby for webbased Artificial Intelligence?
The selection of language is completely irrelevant, all other things being equal.
If you're trying to do X and there's a library for it in language Y and meshes well with your Web-based framework, then use it.
Without knowing more about what specific areas of AI you're interested in, the question is far too vague to be answered with any more specificity.
A new web application may require adding Artificial Intelligence (AI) in the future, e.g. using ProLog. I know it can be done from a Java environment, but I am wondering about the opportunities with modern web languages like Ruby or Python. The latter is considered to be "more scientific" (at least used in that environment), but using Google there seems to be a preliminary ProLog implementation for both. 
Any suggestions on modern (open source) web languages (like Python or Ruby) in combination with AI?