They are all pretty much the same stuff. Find which ever one you can get a good deal on. Bigger is also better. The small ones seem way to small to my eyes.
There are BIG differences between some. The cheap ones typically are lower res, poorer build, low-contrast, poor colour pieces of rubbish.
I work in retail and sell tons of these. The ONE brand that has consistently delivered (still recommend going into a store though) is Kodak. Great contrast, viewing angles, brightness, colour and they almost all support CF (except the new 7" variety). I put my photos on the one at work and it looks clearly as good as my monitor, with maybe *slightly* higher contrast but that is adjustable.
The Sony ones should be good as well if they're anything like their portable dvd players which have great LCD's, 10-bit like their bravia tv's. I may have to order some of their photo frames into the store
Yeah the Sony's rock.
I recently bought the 9" model for my wife. They have quite a wide range of settings from displaying thumbnails to user set time delays between picture changes. Even a combination of image and clock side by side. HDTV output on the 9" for whatever one might use that for.