As mentioned: the answer I give is based on my limited view of ConTeXT's XML processing capabilities. So please edit the answer as I might be wrong.
Input: The input of ConTeXt is mixed TeX and XML. Data is XML, layout (style information) is TeX. The input of speedata is only XML, the layout language is written as XML.
Features: ConTeXt is much older than my software, so there are many more features in ConTeXt than there are in the speedata Publisher. That said: the publisher has enough features to do many typesetting tasks, especially in the direction of “product catalogs”
Virtual pages: ConTeXt can do the virtual boxes as well. This is a basic feature of TeX so there is not much difference here. My software is doing the calculation in grid cells and ConTeXt does the calculation in (scaled) points, but in practice this does not make a difference.
Image shape: I don't know if ConTeXt allows such a thing, I'd appreciate an answer.
My very personal conclusion is: ConTeXt (as LaTeX) is more focused towards text output with complex structures (footnotes, math, ...) and my tool is more focused on the layout aspect. ConTeXt very likely able to create the same layouts as the speedata Publisher.
(Community wiki, anybody please edit this answer and add things)