Customer service is also something to concider when purchasing a fishfinder. I'd advice going with Humminbird. Their customer sevice has been great...I've had a 777c2 on the boat for around 5 years now. I've sent the unit back in a few times for software upgrades and added gps and a barametric pressure sensor. The only issue I've had was loseing the depth readout up at Paulina in a couple areas but fixed it by adjusting the sensitivity.
Look at the 778c HD. This unit goes for around $500. It's color, 4000 watt, 640x640 pixels, and has a dual beam transducer.