GoHarley, Have you looked at the Eagle 320DF yet? it is basically an X-88DF without the flasher feature. Same specs otherwise, 500watt RMS, dual frequency, speed and temp, and 320 x 320 pixels, it is basically a 240 Blue selling for the price of the 160 Blue, $240.
I get asked at work 2 or 3 times a week which is better, Garmin or Lowrance/Eagle, I don't honestly think you can go wrong with either, up until 3 months ago Garmin far outsold everything else but since Lowrance/Eagle came out with their new products (priced to compete with Garmin) I couldn't tell you which is the best seller, its very close.
As for your question on dual frequency, its a must in my boat, single frequency units are generally 200 khz (actually 192khz) and narrow beam, 10 to 20 degree usually which isn't much coverage, dual frequency units give you a 50/200 khz beam that covers alot more area, usually from 40 to 90 degrees depending on brand.
Before you buy, checkout the Eagle.