Autoscan mode imposes an approximate 1 second delay for weak to moderately weak radar signals. Why? Because Autoscan mode inherently filters out pairs of X band door openers. The one second delay is usually enough time, while you are driving, for Autoscan to match up additional microwave door opener signals into pairs of microwave sources without triggering a false alert.
Note that Autoscan has failed on me by occasionally alerting to distant door openers while I am stopped at a busy intersection and my radar detector is detecting multiple random radar reflections from all of the moving cars within the intersection. Nevertheless, Autoscan mode does make for a much quieter drive within suburbs or cities.
When in rural areas or on open highways, obviously highway mode might be preferable since you could be interested in getting the earliest possible alert (even by just 1 additional second of advance warning) in order to potentially avoid an instant-on radar speeding ticket. The problem is that modern instant-on radar guns have become so good that they can measure your speed even if you are decelerating rapidly (stomping on the brakes). So, in short, Highway versus Autoscan isn't likely to make any difference as to whether or not you eventually get nailed by instant-on for speeding.
Thats my two cents!