The 6000 series achieves a waveform update rate of 100,000 waveforms per second. This is actually a metric for how rapidly the scope is acquiring data and posting it to the screen. Therefore, any measurement being made on this rapidly updated signal will have a larger sample space of data as its source, and is thus benefited by such a fast update rate.
on Dec 1, 2006
6000A - 100,000 wfm/s
6000X - 450,000 wfm/s
Here's a video showing the importance of update rate. It's an old one but still valid:
Agilent 7000 Series Oscilloscope: Finding Intermittent Glitches - YouTube