Garmin Gps/sonar Combo Internal Or External Antenna, Dual Or Single Frequency?

I want to buy a new GPS/SONAR for my new boat. Anyone have one with an internal antenna? I would like to know if you ever lose signal.
Whats the difference between the single and double frequency? This is the model Im looking at buying, Eagle SeaCharter 642C DF iGPS but Im interested in the new lowrance hds models also. Any info would be appreciated.