I agree with zridling's comments regarding the fact that a major difference between these search programs is the interface. However, I respectfully disagree with the issue of continually spinning HD's.
With X1, the user determines exactly when, if ever, the index should be updated, either on a scheduled or manually selected time. The initial indexing takes a while but incremental indexes are quite fast.
By the way, a lot of people know about the Indexer that comes with XP (Pro? and maybe even 2k) or later. What most don't seem to know is that it can really be tweaked so that it stays out of your way. Its default settings have it getting in the way of doing anything else on the machine while it's updating. This is made worse by the "tweak" settings being not only left off
of the menus, but they're only on one of two otherwise identical (or nearly identical) context menus.
Just a quick once-over in case it helps some of the folks here: once you go through the Administrative Tools (or Run dialog) to get to the Microsoft Management Console and then the Indexing Service, you right-click the resulting pane on the right, winding your way to this menu:
Then you work your way through a couple of dialogs and you're (finally!) able to customize the performance settings.
Each of the two settings have 3 steps, and with Indexing set to "lazy", and Querying set to "High load", the system then waits for you to have stopped any activity for a period of time before it'll start indexing anything not already in the Catalog. As soon as you start to use the mouse or keyboard, *poof*, it stops and waits for you again. Makes a huge difference.
You may even get good performance with the Indexing at the middle position, but you're guaranteed for it to be
getting in your way
working regardless of your own activities if you set it on Instant, and you can count on a performance hit even with a peppy machine.