Similar to Alexa, which lets you compare website ranks.Feed Compare lets you compare feed subscribers of different sites/blog for comparing them over a period of 1 month,3 months,6 months, 12 months and 24 months for a max of 4 sites at a time.

Just enter the feed name Ex. http://feeds.feedburner.com/feedname in the box, with 3 other feednames for comparison.

You can mouseover the chart for the comparison details, and click and drag for more zooming.

Another alternative for comparing feeds is FeedAnalysis, check out the post about FeedAnalysis here.

This can also be useful to know the number of feed subscribers of a site, as an alternative to a method posted few days backs here.