The important thing to consider with METRo is that it is small in terms of project size. With only one dedicated developer (who is admittedly spends very little time on the project) there is a lot of room to become a 'player' in this project. The problem with the small project size is the maintenance/upgrade demand for this software needs to be marketed and cultivated before it can sustain any dedicated developers. If I were to dedicated a few months of my time towards METRo, I could very well become one of only 5 experts on this code base; if any work were to be commissioned on it, my name would be considered for the commission. This commission idea is a very big if though. Is a large company really going to start demanding maintenance and upgrades to this code-base?

XEN is huge,comprises multiple sub-projects, has lots of developers working professionally on the project. Virtualization is a big and growing marketplace, and XEN has managed to carve itself a nice chunk of market-share. As a huge project XEN is much more difficult to get into than METRo