I actually haven't used Pleco on a 3GS yet - I figure it's better to only use the 3G, that way I'm personally motivated to make sure performance is excellent even on those. Though perhaps a 3GS-equipped beta-tester can chime in on this...
The biggest area that's likely to benefit from a 3GS is the "live mode" web browser feature; it's extremely memory-intensive and the 3GS' extra memory capacity would likely make it a little smoother / less crash-prone than on a regular 3G or a 1st/2nd-gen iPod Touch.
Flashcards might benefit from the faster processor, but we'll have to wait and see on that - it could be that the limiting factor turns out to be file access speed rather than processor speed, in which case there's not likely to be much of a difference in flashcard performance.
Dictionary searches are all pretty much instantaneous on the 3G, so the 3GS isn't likely to make much of a difference with those. I guess it might be possible to turn the "delay before search" from its iPhone default of 0.25 seconds to 0 seconds - the main limiting factor with search results is actually the time it takes to draw them on the screen (rather than the database query, which is virtually instantaneous), so that could be improved by the faster processor to the point where you'd no longer need any delay to avoid blocking the keyboard.
