I prefer the shorter ads when I'm doing the surfing for information. Well-written, succinct ads catch my attention better than the long-winded Yahoo ads (where my eyes often start glazing over) or the short generic ads that don't tell me much of anything.
For example, let's say I'm searching for blue widgets. Here's a bad example of a short ad (usually some type of dynamic keyword insertion):
Check out our supply
Lots in stock!
The above example tells me nothing, and they may not even have anything about blue widgets. Here's a bad example of a longer ad:
Widgets! Red, blue,
green, purple, orange, and more,
and widget accessories in our
electronics and sporting good stores.
Now I'm guessing this site may have information about blue widgets, since they seem to have everything about every type of widget. Or so it seems from the ad. Finding the information for just the blue can take several more clicks, if it even exists on that site.
Here's a good short ad:
New, used, and refurbished
Comparisons and reviews
I'd click that last ad over the first two every time.