LaunchBar lets me choose any track in my iTunes library by typing its name in part. QuickSilver forces me to go to iTunes > Browse Tracks > (type track name), though it then lets me play the track in Party Shuffle while LaunchBar cannot.
LaunchBar lets me visit any URL in any browser: (shortcut key) > CMD+L > type/paste URL > TAB > (browser name). I haven't figured out how to do this in QuickSilver yet.
LaunchBar has been released and is supported. QuickSilver is in perpetual beta mode and succumbs to the open-source mentality of "if a developer shouts you down, you're down." For example, too many QuickSilver users have 20,000 tracks in their iTunes library and as such don't want tracks listed in the main listing.