I would like to question the idea of comparing two completely different applications. There should be some reason to do that which changes from case to case. And this reason should dictate the choice of metrics. So, no we can't have a single universal metric. Though we can use a combination of several metrics to emphasize or support our conclusions.
On the other hand people comparison is much more compelling for managers :) So, for me the real question sounds like "is there a way to compare two developers working on different projects?"
I am totally convinced that it is possible, but again it shouldn't narrow down to some universal metric. Rather everybody chooses what are the most important indicators for him or her.