Tracker hasn't been around as long as Beagle has but seems more advanced, possessing various utilities like the applet which can be used to launch all Tracker GUI utilities (Tracker Search Tool, Tracker Preferences, and Tracker Statistics), displaying the daemon (trackerd) status, providing the functionality of pausing the Tracker daemon, as well as providing a (convenient) search dialogue [update]. I wonder if Beagle has lovely tools like this, but it's main advantage is that it supports far more data formats that Tracker does, while it still is hungry for memory (version 0.3.2).
One particular thing that encouraged this post however is Tracker Statistics, when I discovered that it actually is self-updating. This change, which I apparently missed, was committed October last year, and got me really impressed. Now if only all those advanced options were less buggy...
While I'm at it I might as well mention how much I love the new Tracker Preferences feature which makes it restart/reindex user's data depending on what preferences were changed. That's gorgeous. Now if only it performed such restarting/reindex faster than it often does (especially when it is indexing), since one often has to wait 'forever' for such to happen.