Previews probably supply more buzz for the buck. Faster to create, and they can cover a wider range of topics like John mentioned.
That's not a critique, btw. I like the previews because I see some things that I didn't know were out there, and I'm really glad that they started doing them. I don't pay much attention to the "First Impression" rating, though. Most of the in-depth reviews have limited value to me unless it's a product that I'm in the market for, which is basically never.