Neat! Does it only compare downloads? It's kind of an odd metric to use for finding which one is 'better', but now that Wordpress shows active installs that might be a good view to add.

ie, A plugin with a high download to active install ratio (100 dl's/80 active) would score better than a plugin with a huge amount of downloads and very little active installs (1000 dl's/20 active). Additionally, the age could play a part in calculating that too.

Either way, nice work - I'll be looking forward to seeing some updates!