For me, the BlackBerry is all about text and the 6700 series fits more "readable" text on the screen than the 62XX/72XX series. The low res of the 6710 doesn't bother me when all I do is go through 100s of emails a day. I also like the fact that my 6710 came with a cradle vs. the USB cable for the 62XX/72XX series.
The 7290 screen is just plain dark with the light off. I generally use it in normal office lighting. It looks awesome all lit up.
My perfect device would be a 7790 with auto-backlight.